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/exception_object_alignment.pass.cc

@@ -0,0 +1,39 @@
+//===----------------------------------------------------------------------===//
+//
+// 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-exceptions
+
+// The situation for the alignment of exception objects is badly messed up
+// before macOS 10.14. The test fails on macOS 10.9 to 10.12, passes on macOS
+// 10.13 (no investigation done), and passes afterwards. Just mark all the OSes
+// before 10.14 as unsupported.
+// UNSUPPORTED: stdlib=apple-libc++ && target={{.+}}-apple-macosx10.{{9|10|11|12|13}}
+
+// Check that the pointer __cxa_allocate_exception returns is aligned to the
+// default alignment for the target architecture.
+
+#include "third_party/libcxx/cassert"
+#include "third_party/libcxx/cstdint"
+#include "third_party/libcxxabi/include/cxxabi.h"
+#include "third_party/libcxx/type_traits"
+#include "third_party/libcxxabi/include/__cxxabi_config.h"
+
+struct S {
+  int a[4];
+} __attribute__((aligned));
+
+int main(int, char**) {
+#if !defined(_LIBCXXABI_ARM_EHABI)
+  void *p = __cxxabiv1::__cxa_allocate_exception(16);
+  auto i = reinterpret_cast<uintptr_t>(p);
+  auto a = std::alignment_of<S>::value;
+  assert(i % a == 0);
+  __cxxabiv1::__cxa_free_exception(p);
+#endif
+  return 0;
+}