s390/unwind: always inline get_stack_pointer

Always inline get_stack_pointer() to avoid potential problems
due to compiler inlining decisions, i.e. getting stack pointer of
get_stack_pointer() itself which is later reused.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>