third_party/libcxxabi: Add test suite (#1076)

Added the `libcxxabi` test suite as found in LLVM 17.0.6.

Some tests that do not apply to the current configuration of
comsopolitan are not added. These include:
- `backtrace_test`, `forced_unwind*`: Use unwind function unsupported in
SjLj mode.
- `noexception*`: Designed to test `libcxxabi` in no exceptions mode.

Some tests are added but not enabled due to bugs specific to GCC or
cosmopolitan. These are clearly indicated in the `BUILD.mk` file.

third_party/libcxxabi/test/cxa_thread_atexit_test.pass.cc

@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: no-threads
+// REQUIRES: linux
+
+#include "libc/isystem/assert.h"
+#include "third_party/libcxxabi/include/cxxabi.h"
+
+static bool AtexitImplCalled = false;
+
+extern "C" int __cxa_thread_atexit_impl(void (*dtor)(void *), void *obj,
+                                        void *dso_symbol) {
+  assert(dtor == reinterpret_cast<void (*)(void *)>(1));
+  assert(obj == reinterpret_cast<void *>(2));
+  assert(dso_symbol == reinterpret_cast<void *>(3));
+  AtexitImplCalled = true;
+  return 4;
+}
+
+int main(int, char**) {
+  int RV = __cxxabiv1::__cxa_thread_atexit(
+      reinterpret_cast<void (*)(void *)>(1), reinterpret_cast<void *>(2),
+      reinterpret_cast<void *>(3));
+  assert(RV == 4);
+  assert(AtexitImplCalled);
+  return 0;
+}