libstdc++: Add -pthread options to std::future polling test

For linux targets this test doesn't need -lpthread because it only uses
atomics, but for all other targets std::call_once still needs pthreads.
Add the necessary test directives to make that work.

The timings in this test might be too fragile or too target-specific, so
it might need to be adjusted in future, or restricted to only run on
specific targets. For now I've increased the allowed ratio between
wait_for calls before and after the future is made ready, because it was
failing with -O3 -march=native sometimes.

libstdc++-v3/ChangeLog:

	* testsuite/30_threads/future/members/poll.cc: Require gthreads
	and add -pthread for targets that require it. Relax required
	ratio of wait_for calls before/after the future is ready.