support: Add xclock_gettime

	* support/xclock_gettime.c (xclock_gettime): New file. Provide
	clock_gettime wrapper for use in tests that fails the test rather
	than returning failure.

	* support/xtime.h: New file to declare xclock_gettime.

	* support/Makefile: Add xclock_gettime.c.

	* support/README: Mention xtime.h.