s390/smp: Implement raw_smp_processor_id() with inline assembly

Implement raw_smp_processor_id() with an inline assembly, which makes
use of the ALTERNATIVE macro, to read cpu_nr from lowcore. Provide an
alternative instruction with a different offset in case lowcore is
relocated.

This replaces sequences of two instructions with one instruction.

Before:
  1000b6:       a5 1e 00 00             llilh   %r1,0
  1000ba:       58 20 13 a0             l       %r2,928(%r1)

After:
  1000b6:       e3 20 03 a0 00 58       ly      %r2,928

Kernel image size change:
add/remove: 753/755 grow/shrink: 230/1510 up/down: 30538/-35832 (-5294)

Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>