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_handlers.h

@@ -0,0 +1,55 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 functionality associated with the terminate_handler,
+//   unexpected_handler, and new_handler.
+//===----------------------------------------------------------------------===//
+
+#ifndef _CXA_HANDLERS_H
+#define _CXA_HANDLERS_H
+
+#include "third_party/libcxxabi/include/__cxxabi_config.h"
+
+#include "third_party/libcxx/exception"
+
+namespace std
+{
+
+_LIBCXXABI_HIDDEN _LIBCXXABI_NORETURN
+void
+__unexpected(unexpected_handler func);
+
+_LIBCXXABI_HIDDEN _LIBCXXABI_NORETURN
+void
+__terminate(terminate_handler func) noexcept;
+
+}  // std
+
+extern "C"
+{
+
+_LIBCXXABI_DATA_VIS extern void (*__cxa_terminate_handler)();
+_LIBCXXABI_DATA_VIS extern void (*__cxa_unexpected_handler)();
+_LIBCXXABI_DATA_VIS extern void (*__cxa_new_handler)();
+
+/*
+
+    At some point in the future these three symbols will become
+    C++11 atomic variables:
+
+    extern std::atomic<std::terminate_handler>  __cxa_terminate_handler;
+    extern std::atomic<std::unexpected_handler> __cxa_unexpected_handler;
+    extern std::atomic<std::new_handler>        __cxa_new_handler;
+
+    This change will not impact their ABI.  But it will allow for a
+    portable performance optimization.
+
+*/
+
+} // extern "C"
+
+#endif // _CXA_HANDLERS_H