s390/stack: merge empty stack frame slots

Merge empty1 and empty2 arrays within the stack frame to one single
array. This is possible since with commit 42b01a55

 ("s390: always
use the packed stack layout") the alternative stack frame layout is
gone.

Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>