s390/cpum_sf: Add helper to read TOD from trailer entries

The trailer entry contains a timestamp of the time when the sample-data-block
became full.  The timestamp specifies a TOD (time-of-day) value in either the
STCK or STCKE format.

Provide a helper function to return the TOD value depending on the setting of
time format indicator.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>