[S390] Introduce get_clock_fast()

Add get_clock_fast() which uses the slightly faster stckf if available.
If stckf is not available fall back to stck, which has the same width.

Signed-off-by: Jan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>