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.2.pass.cc

@@ -0,0 +1,34 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 test checks that the compiler does not make incorrect assumptions
+// about the alignment of the exception (only in that specific case, of
+// course).
+//
+// There was a bug where Clang would emit a call to memset assuming a 16-byte
+// aligned exception even when back-deploying to older Darwin systems where
+// exceptions are 8-byte aligned, which caused a segfault on those systems.
+
+struct exception {
+    exception() : x(0) { }
+    virtual ~exception() { }
+    int x;
+};
+
+struct foo : exception { };
+
+int main(int, char**) {
+    try {
+      throw foo();
+    } catch (...) {
+
+    }
+    return 0;
+}