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_pointer_nullptr.pass.cc

@@ -0,0 +1,81 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// Catching an exception thrown as nullptr was not properly handled before
+// 2f984cab4fa7, which landed in macOS 10.13
+// XFAIL: stdlib=apple-libc++ && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
+// UNSUPPORTED: c++03
+// UNSUPPORTED: no-exceptions
+
+#include "third_party/libcxx/cassert"
+#include "third_party/libcxx/cstdlib"
+
+struct A {};
+
+void test1()
+{
+    try
+    {
+        throw nullptr;
+        assert(false);
+    }
+    catch (int* p)
+    {
+        assert(!p);
+    }
+    catch (long*)
+    {
+        assert(false);
+    }
+}
+
+void test2()
+{
+    try
+    {
+        throw nullptr;
+        assert(false);
+    }
+    catch (A* p)
+    {
+        assert(!p);
+    }
+    catch (int*)
+    {
+        assert(false);
+    }
+}
+
+template <class Catch>
+void catch_nullptr_test() {
+  try {
+    throw nullptr;
+    assert(false);
+  } catch (Catch c) {
+    assert(!c);
+  } catch (...) {
+    assert(false);
+  }
+}
+
+
+int main(int, char**)
+{
+  // catch naked nullptrs
+  test1();
+  test2();
+
+  catch_nullptr_test<int*>();
+  catch_nullptr_test<int**>();
+  catch_nullptr_test<int A::*>();
+  catch_nullptr_test<const int A::*>();
+  catch_nullptr_test<int A::**>();
+
+  return 0;
+}