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/cxa_aux_runtime.cc

@@ -0,0 +1,44 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//
+// This file implements the "Auxiliary Runtime APIs"
+// https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html#cxx-aux
+//===----------------------------------------------------------------------===//
+
+#include "third_party/libcxxabi/include/cxxabi.h"
+#include "third_party/libcxx/exception"
+#include "third_party/libcxx/new"
+#include "third_party/libcxx/typeinfo"
+
+namespace __cxxabiv1 {
+extern "C" {
+_LIBCXXABI_FUNC_VIS _LIBCXXABI_NORETURN void __cxa_bad_cast(void) {
+#ifndef _LIBCXXABI_NO_EXCEPTIONS
+  throw std::bad_cast();
+#else
+  std::terminate();
+#endif
+}
+
+_LIBCXXABI_FUNC_VIS _LIBCXXABI_NORETURN void __cxa_bad_typeid(void) {
+#ifndef _LIBCXXABI_NO_EXCEPTIONS
+  throw std::bad_typeid();
+#else
+  std::terminate();
+#endif
+}
+
+_LIBCXXABI_FUNC_VIS _LIBCXXABI_NORETURN void
+__cxa_throw_bad_array_new_length(void) {
+#ifndef _LIBCXXABI_NO_EXCEPTIONS
+  throw std::bad_array_new_length();
+#else
+  std::terminate();
+#endif
+}
+} // extern "C"
+} // abi