libstdc++: Add "futex" and "gthreads" effective-target keywords

libstdc++: Add "futex" and "gthreads" effective-target keywords

This adds a new "futex" effective-target keyword that can be used to
selectively enable/disable tests based on _GLIBCXX_HAVE_LINUX_FUTEX,
instead of checking for that macro in the code.

It also adds "gthreads" as another one, to make the result of the
dg-require-gthreads directive usable in target selectors.

With these new keywords two tests that are currently only run for linux
can also be run for targets using gthr-single.h (e.g. AIX single-thread
multilib, and targets without a gthreads implementation).

libstdc++-v3/ChangeLog:

	* testsuite/18_support/96817.cc: Use new effective-target
	keywords to select supported targets more effectively.
	* testsuite/30_threads/call_once/66146.cc: Likewise.
	* testsuite/lib/libstdc++.exp (check_effective_target_futex):
	Define new proc.
	(check_effective_target_gthreads): Define new proc to replace
	dg-require-gthreads.