Copy vendor/llvm/default_assertion_handler.in to __assertion_handler

Our prevous `__assertion_handler` was copied from
`libcxx/vendor/llvm/default_assertion_handler.in`. This updates out
`__assertion_handler` to the new
`libcxx/vendor/llvm/default_assertion_handler.in`. For what this file
is, see the PR description of emscripten-core#22994, with which the file was added.

Author: aheejin

system/lib/libcxx/include/__assertion_handler

@@ -10,8 +10,13 @@
 #ifndef _LIBCPP___ASSERTION_HANDLER
 #define _LIBCPP___ASSERTION_HANDLER
 
-#include <__config>
-#include <__verbose_abort>
+#if __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
+#  include <__cxx03/__config>
+#  include <__cxx03/__verbose_abort>
+#else
+#  include <__config>
+#  include <__verbose_abort>
+#endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header