nptl: Convert some rwlock tests to use libsupport

Checked on x86_64-linux-gnu and i686-linux-gnu.

	* nptl/tst-rwlock6.c: Use libsupport. This also happens to fix a
	small bug where only tv.tv_usec was checked which could cause an
	erroneous pass if pthread_rwlock_timedrdlock incorrectly took more
	than a second.

	* nptl/tst-rwlock7.c, nptl/tst-rwlock9.c, nptl/tst-rwlock14.c: Use
	libsupport.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>