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/CREDITS.TXT

@@ -0,0 +1,71 @@
+This file is a partial list of people who have contributed to the LLVM/libc++abi
+project.  If you have contributed a patch or made some other contribution to
+LLVM/libc++abi, please submit a patch to this file to add yourself, and it will be
+done!
+
+The list is sorted by surname and formatted to allow easy grepping and
+beautification by scripts.  The fields are: name (N), email (E), web-address
+(W), PGP key ID and fingerprint (P), description (D), and snail-mail address
+(S).
+
+N: Aaron Ballman
+E: aaron@aaronballman.com
+D: Minor patches
+
+N: Logan Chien
+E: logan.chien@mediatek.com
+D: ARM EHABI Unwind & Exception Handling
+
+N: Marshall Clow
+E: mclow.lists@gmail.com
+E: marshall@idio.com
+D: Architect and primary coauthor of libc++abi
+
+N: Matthew Dempsky
+E: matthew@dempsky.org
+D: Minor patches and bug fixes.
+
+N: Nowar Gu
+E: wenhan.gu@gmail.com
+D: Minor patches and fixes
+
+N: Howard Hinnant
+E: hhinnant@apple.com
+D: Architect and primary coauthor of libc++abi
+
+N: Dana Jansens
+E: danakj@chromium.org
+D: ARM EHABI Unwind & Exception Handling
+
+N: Nick Kledzik
+E: kledzik@apple.com
+
+N: Antoine Labour
+E: piman@chromium.org
+D: ARM EHABI Unwind & Exception Handling
+
+N: Bruce Mitchener, Jr.
+E: bruce.mitchener@gmail.com
+D: Minor typo fixes
+
+N: Andrew Morrow
+E: andrew.c.morrow@gmail.com
+D: Minor patches and fixes
+
+N: Erik Olofsson
+E: erik.olofsson@hansoft.se
+E: erik@olofsson.info
+D: Minor patches and fixes
+
+N: Jon Roelofs
+E: jroelofs@jroelofs.com
+D: ARM EHABI Unwind & Exception Handling, Bare-metal
+
+N: Nico Weber
+E: thakis@chromium.org
+D: ARM EHABI Unwind & Exception Handling
+
+N: Albert J. Wong
+E: ajwong@google.com
+D: ARM EHABI Unwind & Exception Handling
+