Add default __assertion_handler to libcxx/include

llvm/llvm-project#77883 adds a way for vendors
to override how we handle assertions. If a vendor doesn't want to
override it, `CMakeLists.txt` will copy the provided default template
assertion handler file
(https://github.com/llvm/llvm-project/blob/26a1d6601d727a96f4301d0d8647b5a42760ae0c/libcxx/vendor/llvm/default_assertion_handler.in)
to libcxx's generated include dir, which defaults to
`SYSROOT/include/c++/v1`:
https://github.com/llvm/llvm-project/blob/26a1d6601d727a96f4301d0d8647b5a42760ae0c/libcxx/CMakeLists.txt#L72-L73
https://github.com/llvm/llvm-project/blob/26a1d6601d727a96f4301d0d8647b5a42760ae0c/libcxx/CMakeLists.txt#L439
https://github.com/llvm/llvm-project/blob/26a1d6601d727a96f4301d0d8647b5a42760ae0c/libcxx/include/CMakeLists.txt#L1024

We don't use CMake, so this renames the provided
`vendor/llvm/default_assertion_handler.in` to `__assert_handler` in our
`include` directory, which will be copied to `SYSROOT/include/c++/v1`.

system/lib/libcxx/include/__assertion_handler

@@ -0,0 +1,31 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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 _LIBCPP___ASSERTION_HANDLER
+#define _LIBCPP___ASSERTION_HANDLER
+
+#include <__config>
+#include <__verbose_abort>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
+
+#  define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_VERBOSE_ABORT("%s", message)
+
+#else
+
+// TODO(hardening): use `__builtin_verbose_trap(message)` once that becomes available.
+#  define _LIBCPP_ASSERTION_HANDLER(message) ((void)message, __builtin_trap())
+
+#endif // _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
+
+#endif // _LIBCPP___ASSERTION_HANDLER