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/include/__cxxabi_config.h

@@ -0,0 +1,106 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef ____CXXABI_CONFIG_H
+#define ____CXXABI_CONFIG_H
+
+#if defined(__arm__) && !defined(__USING_SJLJ_EXCEPTIONS__) &&                 \
+    !defined(__ARM_DWARF_EH__) && !defined(__SEH__)
+#define _LIBCXXABI_ARM_EHABI
+#endif
+
+#if !defined(__has_attribute)
+#define __has_attribute(_attribute_) 0
+#endif
+
+#if defined(__clang__)
+#  define _LIBCXXABI_COMPILER_CLANG
+#  ifndef __apple_build_version__
+#    define _LIBCXXABI_CLANG_VER (__clang_major__ * 100 + __clang_minor__)
+#  endif
+#elif defined(__GNUC__)
+#  define _LIBCXXABI_COMPILER_GCC
+#elif defined(_MSC_VER)
+#  define _LIBCXXABI_COMPILER_MSVC
+#elif defined(__IBMCPP__)
+#  define _LIBCXXABI_COMPILER_IBM
+#endif
+
+#if defined(_WIN32)
+ #if defined(_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS) || (defined(__MINGW32__) && !defined(_LIBCXXABI_BUILDING_LIBRARY))
+  #define _LIBCXXABI_HIDDEN
+  #define _LIBCXXABI_DATA_VIS
+  #define _LIBCXXABI_FUNC_VIS
+  #define _LIBCXXABI_TYPE_VIS
+ #elif defined(_LIBCXXABI_BUILDING_LIBRARY)
+  #define _LIBCXXABI_HIDDEN
+  #define _LIBCXXABI_DATA_VIS __declspec(dllexport)
+  #define _LIBCXXABI_FUNC_VIS __declspec(dllexport)
+  #define _LIBCXXABI_TYPE_VIS __declspec(dllexport)
+ #else
+  #define _LIBCXXABI_HIDDEN
+  #define _LIBCXXABI_DATA_VIS __declspec(dllimport)
+  #define _LIBCXXABI_FUNC_VIS __declspec(dllimport)
+  #define _LIBCXXABI_TYPE_VIS __declspec(dllimport)
+ #endif
+#else
+ #if !defined(_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS)
+  #define _LIBCXXABI_HIDDEN __attribute__((__visibility__("hidden")))
+  #define _LIBCXXABI_DATA_VIS __attribute__((__visibility__("default")))
+  #define _LIBCXXABI_FUNC_VIS __attribute__((__visibility__("default")))
+  #if __has_attribute(__type_visibility__)
+   #define _LIBCXXABI_TYPE_VIS __attribute__((__type_visibility__("default")))
+  #else
+   #define _LIBCXXABI_TYPE_VIS __attribute__((__visibility__("default")))
+  #endif
+ #else
+  #define _LIBCXXABI_HIDDEN
+  #define _LIBCXXABI_DATA_VIS
+  #define _LIBCXXABI_FUNC_VIS
+  #define _LIBCXXABI_TYPE_VIS
+ #endif
+#endif
+
+#if defined(_LIBCXXABI_COMPILER_MSVC)
+#define _LIBCXXABI_WEAK
+#else
+#define _LIBCXXABI_WEAK __attribute__((__weak__))
+#endif
+
+#if defined(__clang__)
+#define _LIBCXXABI_COMPILER_CLANG
+#elif defined(__GNUC__)
+#define _LIBCXXABI_COMPILER_GCC
+#endif
+
+#if __has_attribute(__no_sanitize__) && defined(_LIBCXXABI_COMPILER_CLANG)
+#define _LIBCXXABI_NO_CFI __attribute__((__no_sanitize__("cfi")))
+#else
+#define _LIBCXXABI_NO_CFI
+#endif
+
+// wasm32 follows the arm32 ABI convention of using 32-bit guard.
+#if defined(__arm__) || defined(__wasm32__) || defined(__ARM64_ARCH_8_32__)
+#  define _LIBCXXABI_GUARD_ABI_ARM
+#endif
+
+#if defined(_LIBCXXABI_COMPILER_CLANG)
+#  if !__has_feature(cxx_exceptions)
+#    define _LIBCXXABI_NO_EXCEPTIONS
+#  endif
+#elif defined(_LIBCXXABI_COMPILER_GCC) && !defined(__EXCEPTIONS)
+#  define _LIBCXXABI_NO_EXCEPTIONS
+#endif
+
+#if defined(_WIN32)
+#define _LIBCXXABI_DTOR_FUNC __thiscall
+#else
+#define _LIBCXXABI_DTOR_FUNC
+#endif
+
+#endif // ____CXXABI_CONFIG_H