libstdc++: Fix std::format test for strict -std=c++20 mode

Adjust a test to avoid using std::make_unsigned_t<__int128>. That's
ill-formed in strict modes because std::is_integral_v<__int128> is
false.

libstdc++-v3/ChangeLog:

	* testsuite/std/format/functions/format.cc: Do not use
	std::make_unsigned_t<__int128>.