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/catch_in_noexcept.pass.cc

@@ -0,0 +1,38 @@
+//===---------------------- catch_in_noexcept.cpp--------------------------===//
+//
+// 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: c++03
+// UNSUPPORTED: no-exceptions
+
+#include "third_party/libcxx/exception"
+#include "libc/isystem/stdlib.h"
+#include "libc/isystem/assert.h"
+
+struct A {};
+
+// Despite being marked as noexcept, this function must have an EHT entry that
+// is not 'cantunwind', so that the unwinder can correctly deal with the throw.
+void f1() noexcept
+{
+    try {
+        A a;
+        throw a;
+        assert(false);
+    } catch (...) {
+        assert(true);
+        return;
+    }
+    assert(false);
+}
+
+int main(int, char**)
+{
+    f1();
+
+    return 0;
+}