libstdc++: Fix FS-dependent filesystem tests

libstdc++: Fix FS-dependent filesystem tests

These tests were failing on XFS because it doesn't support setting file
timestamps past 2038, so the expected overflow when reading back a huge
timestamp into a file_time_type didn't happen.

Additionally, the std::filesystem::file_time_type::clock has an
epoch that is out of range of 32-bit time_t so testing times around that
epoch may also fail.

This fixes the tests to give up gracefully if the filesystem doesn't
support times that can't be represented in 32-bit time_t.

	* testsuite/27_io/filesystem/operations/last_write_time.cc: Fixes for
	filesystems that silently truncate timestamps.
	* testsuite/experimental/filesystem/operations/last_write_time.cc:
	Likewise.