s390: move on_thread_stack() to processor.h

As preparation for the stackleak feature move on_thread_stack() to
processor.h like x86.

Also make it __always_inline, and slightly optimize it by reading
current task's kernel stack pointer from lowcore.

Reviewed-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>