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/uncaught_exception.pass.cc

@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// This tests that libc++abi still provides __cxa_uncaught_exception() for
+// ABI compatibility, even though the Standard doesn't require it to.
+
+#include "third_party/libcxxabi/include/cxxabi.h"
+#include "third_party/libcxx/cassert"
+
+// namespace __cxxabiv1 {
+//      extern bool __cxa_uncaught_exception () throw();
+// }
+
+struct A {
+    ~A() { assert( __cxxabiv1::__cxa_uncaught_exception()); }
+};
+
+int main () {
+    try { A a; throw 3; assert(false); }
+    catch (int) {}
+}