Make these functions restore the default handlers when passed a null
pointer. This is consistent with std::pmr::set_default_resource(0), and
also matches the current behaviour of libc++.

In order to avoid duplicating the preprocessor condition from
eh_term_handler.cc more that into a new eh_term_handler.h header and
define a macro that can be used in both eh_term_handler.cc and
eh_terminate.cc.

	PR libstdc++/90682
	* libsupc++/eh_term_handler.cc: Include eh_term_handler.h to get
	definition of _GLIBCXX_DEFAULT_TERM_HANDLER.
	* libsupc++/eh_term_handler.h: New header defining
	_GLIBCXX_DEFAULT_TERM_HANDLER.
	* libsupc++/eh_terminate.cc: Include eh_term_handler.h.
	(set_terminate): Restore default handler when argument is null.
	(set_unexpected): Likewise.
	* testsuite/18_support/set_terminate.cc: New test.
	* testsuite/18_support/set_unexpected.cc: New test.

Tested x86_64-linux, committed to trunk.