Add LLVM libcxxabi (#1063)

* third_party: Add libcxxabi

Added libcxxabi from LLVM 17.0.6
The library implements the Itanium C++ exception handling ABI.

* third_party/libcxxabi: Enable __cxa_thread_atexit

Enable `__cxa_thread_atexit` from libcxxabi.
`__cxa_thread_atexit_impl` is still implemented by the cosmo libc.
The original `__cxa_thread_atexit` has been removed.

* third_party/libcxx: Build with exceptions

Build libcxx with exceptions enabled.

- Removed `_LIBCPP_NO_EXCEPTIONS` from `__config`.
- Switched the exception implementation to `libcxxabi`. These two files
are taken from the same `libcxx` version as mentioned in `README.cosmo`.
- Removed `new_handler_fallback` in favor of `libcxxabi` implementation.
- Enable `-fexceptions` and `-frtti` for `libcxx`.
- Removed `THIRD_PARTY_LIBCXX` dependency from `libcxxabi` and
`libunwind`. These libraries do not use any runtime `libcxx` functions,
just headers.

* libc: Remove remaining redundant cxa functions

- `__cxa_pure_virtual` in `libcxxabi` is also a stub similar to the
existing one.
- `__cxa_guard_*` from `libcxxabi` is used instead of the ones from
Android.

Now there should be no more duplicate implementations.
`__cxa_thread_atexit_impl`, `__cxa_atexit`, and related supporting
functions, are still left to other libraries as in `libcxxabi`.

`libcxxabi` is also now added to `cosmopolitan.a` to make up for the
removed functions.

Affected in-tree libraries (`third_party/double-conversion`) have been
updated.

third_party/libcxxabi/BUILD.mk

@@ -0,0 +1,112 @@
+#-*-mode:makefile-gmake;indent-tabs-mode:t;tab-width:8;coding:utf-8-*-┐
+#── vi: set noet ft=make ts=8 sw=8 fenc=utf-8 :vi ────────────────────┘
+
+PKGS += THIRD_PARTY_LIBCXXABI
+
+THIRD_PARTY_LIBCXXABI_ARTIFACTS += THIRD_PARTY_LIBCXXABI_A
+THIRD_PARTY_LIBCXXABI = $(THIRD_PARTY_LIBCXXABI_A_DEPS) $(THIRD_PARTY_LIBCXXABI_A)
+THIRD_PARTY_LIBCXXABI_A = o/$(MODE)/third_party/libcxxabi/libcxxabi.a
+
+THIRD_PARTY_LIBCXXABI_A_INCS =						\
+	third_party/libcxxabi/demangle/ItaniumNodes.def			\
+	third_party/libcxxabi/aix_state_tab_eh.inc			\
+	third_party/libcxxabi/cxa_exception.h				\
+	third_party/libcxxabi/cxa_guard_impl.h				\
+	third_party/libcxxabi/cxa_handlers.h
+
+# TODO: Remove this aligned_alloc header when libcxx is updated
+THIRD_PARTY_LIBCXXABI_A_HDRS =						\
+	third_party/libcxxabi/libcxx/include/__memory/aligned_alloc.h	\
+	third_party/libcxxabi/include/__cxxabi_config.h			\
+	third_party/libcxxabi/include/cxxabi.h				\
+	third_party/libcxxabi/demangle/DemangleConfig.h			\
+	third_party/libcxxabi/demangle/ItaniumDemangle.h		\
+	third_party/libcxxabi/demangle/StringViewExtras.h		\
+	third_party/libcxxabi/demangle/Utility.h			\
+	third_party/libcxxabi/abort_message.h				\
+	third_party/libcxxabi/fallback_malloc.h				\
+	third_party/libcxxabi/private_typeinfo.h
+
+THIRD_PARTY_LIBCXXABI_A_SRCS_CC =					\
+	third_party/libcxxabi/abort_message.cc				\
+	third_party/libcxxabi/cxa_aux_runtime.cc			\
+	third_party/libcxxabi/cxa_default_handlers.cc			\
+	third_party/libcxxabi/cxa_demangle.cc				\
+	third_party/libcxxabi/cxa_exception_storage.cc			\
+	third_party/libcxxabi/cxa_exception.cc				\
+	third_party/libcxxabi/cxa_guard.cc				\
+	third_party/libcxxabi/cxa_handlers.cc				\
+	third_party/libcxxabi/cxa_personality.cc			\
+	third_party/libcxxabi/cxa_thread_atexit.cc			\
+	third_party/libcxxabi/cxa_vector.cc				\
+	third_party/libcxxabi/cxa_virtual.cc				\
+	third_party/libcxxabi/fallback_malloc.cc			\
+	third_party/libcxxabi/private_typeinfo.cc			\
+	third_party/libcxxabi/stdlib_exception.cc			\
+	third_party/libcxxabi/stdlib_new_delete.cc			\
+	third_party/libcxxabi/stdlib_stdexcept.cc			\
+	third_party/libcxxabi/stdlib_typeinfo.cc
+
+THIRD_PARTY_LIBCXXABI_A_SRCS =						\
+	$(THIRD_PARTY_LIBCXXABI_A_SRCS_CC)
+
+THIRD_PARTY_LIBCXXABI_A_OBJS =						\
+	$(THIRD_PARTY_LIBCXXABI_A_SRCS_CC:%.cc=o/$(MODE)/%.o)
+
+THIRD_PARTY_LIBCXXABI_A_CHECKS =					\
+	$(THIRD_PARTY_LIBCXXABI_A).pkg					\
+	$(THIRD_PARTY_LIBCXXABI_A_HDRS:%=o/$(MODE)/%.okk)
+
+THIRD_PARTY_LIBCXXABI_A_DIRECTDEPS =					\
+	LIBC_CALLS							\
+	LIBC_INTRIN							\
+	LIBC_MEM							\
+	LIBC_NEXGEN32E							\
+	LIBC_RUNTIME							\
+	LIBC_STDIO							\
+	LIBC_STR							\
+	LIBC_THREAD							\
+	THIRD_PARTY_LIBUNWIND
+
+THIRD_PARTY_LIBCXXABI_A_DEPS :=						\
+	$(call uniq,$(foreach x,$(THIRD_PARTY_LIBCXXABI_A_DIRECTDEPS),$($(x))))
+
+$(THIRD_PARTY_LIBCXXABI_A):						\
+		third_party/libcxxabi/					\
+		$(THIRD_PARTY_LIBCXXABI_A).pkg				\
+		$(THIRD_PARTY_LIBCXXABI_A_OBJS)
+
+$(THIRD_PARTY_LIBCXXABI_A).pkg:						\
+		$(THIRD_PARTY_LIBCXXABI_A_OBJS)				\
+		$(foreach x,$(THIRD_PARTY_LIBCXXABI_A_DIRECTDEPS),$($(x)_A).pkg)
+
+# TODO: Remove constinit hacks when we have C++20
+$(THIRD_PARTY_LIBCXXABI_A_OBJS): private				\
+		CXXFLAGS +=						\
+			-ffunction-sections				\
+			-fdata-sections					\
+			-fexceptions					\
+			-frtti						\
+			-D_LIBCXXABI_BUILDING_LIBRARY			\
+			-D_LIBCPP_BUILDING_LIBRARY			\
+			-DHAVE___CXA_THREAD_ATEXIT_IMPL			\
+			-D_LIBCPP_CONSTINIT=__constinit			\
+			-Dconstinit=__constinit
+
+# ttypeIndex is initialized and used only when native_old_exception is true.
+# GCC does not seem to allow this.
+o/$(MODE)/third_party/libcxxabi/cxa_personality.o: private		\
+		CXXFLAGS +=						\
+			-Wno-error=maybe-uninitialized
+
+THIRD_PARTY_LIBCXXABI_LIBS = $(foreach x,$(THIRD_PARTY_LIBCXXABI_ARTIFACTS),$($(x)))
+THIRD_PARTY_LIBCXXABI_SRCS = $(foreach x,$(THIRD_PARTY_LIBCXXABI_ARTIFACTS),$($(x)_SRCS))
+THIRD_PARTY_LIBCXXABI_HDRS = $(foreach x,$(THIRD_PARTY_LIBCXXABI_ARTIFACTS),$($(x)_HDRS))
+THIRD_PARTY_LIBCXXABI_INCS = $(foreach x,$(THIRD_PARTY_LIBCXXABI_ARTIFACTS),$($(x)_INCS))
+THIRD_PARTY_LIBCXXABI_CHECKS = $(foreach x,$(THIRD_PARTY_LIBCXXABI_ARTIFACTS),$($(x)_CHECKS))
+THIRD_PARTY_LIBCXXABI_OBJS = $(foreach x,$(THIRD_PARTY_LIBCXXABI_ARTIFACTS),$($(x)_OBJS))
+
+.PHONY: o/$(MODE)/third_party/libcxxabi
+o/$(MODE)/third_party/libcxxabi: \
+	$(THIRD_PARTY_LIBCXXABI_CHECKS)	\
+	$(THIRD_PARTY_LIBCXXABI_A)