s390/cpum_cf: add ctr_stcctm() function

Introduce the ctr_stcctm() function as wrapper function to extract counters
from a particular counter set.  Note that the counter set is part of the
stcctm instruction opcode, few indirections are necessary to specify the
counter set as variable.

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