selftests/x86: Add clock_gettime() tests to test_vdso

Now that the vDSO implementation of clock_gettime() is getting
reworked, add a selftest for it.  This tests that its output is
consistent with the syscall version.

This is marked for stable to serve as a test for commit

  715bd9d1 ("x86/vdso: Fix asm constraints on vDSO syscall fallbacks")

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/082399674de2619b2befd8c0dde49b260605b126.1538422295.git.luto@kernel.org