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/dynamic_cast_stress.pass.cc

@@ -0,0 +1,82 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+#include "third_party/libcxx/cassert"
+#include "third_party/libcxx/tuple"
+#include "third_party/libcxxabi/test/support/timer.hh"
+
+template <std::size_t Indx, std::size_t Depth>
+struct C
+    : public virtual C<Indx, Depth-1>,
+      public virtual C<Indx+1, Depth-1>
+{
+    virtual ~C() {}
+};
+
+template <std::size_t Indx>
+struct C<Indx, 0>
+{
+    virtual ~C() {}
+};
+
+template <std::size_t Indx, std::size_t Depth>
+struct B
+    : public virtual C<Indx, Depth-1>,
+      public virtual C<Indx+1, Depth-1>
+{
+};
+
+template <class Indx, std::size_t Depth>
+struct makeB;
+
+template <std::size_t ...Indx, std::size_t Depth>
+struct makeB<std::__tuple_indices<Indx...>, Depth>
+    : public B<Indx, Depth>...
+{
+};
+
+template <std::size_t Width, std::size_t Depth>
+struct A
+    : public makeB<typename std::__make_tuple_indices<Width>::type, Depth>
+{
+};
+
+void test()
+{
+    const std::size_t Width = 10;
+    const std::size_t Depth = 5;
+    A<Width, Depth> a;
+    typedef B<Width/2, Depth> Destination;
+//    typedef A<Width, Depth> Destination;
+    Destination *b = nullptr;
+    {
+        timer t;
+        b = dynamic_cast<Destination*>((C<Width/2, 0>*)&a);
+    }
+    assert(b != 0);
+}
+
+int main(int, char**)
+{
+    test();
+
+    return 0;
+}
+
+/*
+Timing results I'm seeing (median of 3 microseconds):
+
+                          libc++abi    gcc's dynamic_cast
+B<Width/2, Depth> -O3      48.334         93.190           libc++abi 93% faster
+B<Width/2, Depth> -Os      58.535         94.103           libc++abi 61% faster
+A<Width, Depth>   -O3      11.515         33.134           libc++abi 188% faster
+A<Width, Depth>   -Os      12.631         31.553           libc++abi 150% faster
+
+*/