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/support/timer.hh

@@ -0,0 +1,54 @@
+//===----------------------------------------------------------------------===////
+//
+// 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
+//
+//===----------------------------------------------------------------------===////
+
+#ifndef TIMER_H
+#define TIMER_H
+
+// Define LIBCXXABI_USE_TIMER to enable testing with a timer.
+#if defined(LIBCXXABI_USE_TIMER)
+
+#include "third_party/libcxx/chrono"
+#include "third_party/libcxx/cstdio"
+
+class timer
+{
+    typedef std::chrono::high_resolution_clock Clock;
+    typedef Clock::time_point TimePoint;
+    typedef std::chrono::microseconds MicroSeconds;
+public:
+    timer() : m_start(Clock::now()) {}
+
+    timer(timer const &) = delete;
+    timer & operator=(timer const &) = delete;
+
+    ~timer()
+    {
+        using std::chrono::duration_cast;
+        TimePoint end = Clock::now();
+        MicroSeconds us = duration_cast<MicroSeconds>(end - m_start);
+        std::printf("%d microseconds\n", us.count());
+    }
+
+private:
+    TimePoint m_start;
+};
+
+#else /* LIBCXXABI_USE_TIMER */
+
+class timer
+{
+public:
+    timer() {}
+    timer(timer const &) = delete;
+    timer & operator=(timer const &) = delete;
+    ~timer() {}
+};
+
+#endif /* LIBCXXABI_USE_TIMER */
+
+#endif /* TIMER_H */