libstdc++: testsuite: skip fs last_write_time tests if dummy

libstdc++: testsuite: skip fs last_write_time tests if dummy

The last_write_time functions are defined in ways that are useful, or
that fail immediately, depending on various macros.  When they fail
immediately, the filesystem last_write_time.cc tests fail noisily, but
the fail is entirely expected.

Define NO_LAST_WRITE_TIME in the last_write_time.cc tests, according
to the macros that select implementations of last_write_time, and use
it through the new dg-require-target-fs-lwt to skip tests that are
expected to fail.

for  libstdc++-v3/ChangeLog

	* testsuite/util/testsuite_fs.h (NO_LAST_WRITE_TIME): Define
	when appropriate.
	* testsuite/lib/libstdc++.exp
	(check_v3_target_fs_last_write_time): New.
	* testsuite/lib/dg-options.exp (dg-require-target-fs-lwt):
	New.
	* testsuite/27_io/filesystem/operations/last_write_time.cc:
	Skip the test if the features are unavailable.
	* testsuite/experimental/filesystem/operations/last_write_time.cc:
	Likewise.