s390: enable DEBUG_FORCE_FUNCTION_ALIGN_64B

Allow to enforce 64 byte function alignment like it is possible for a
couple of other architectures. This may or may not be helpful for
debugging performance problems, as described with the Kconfig option.

Since the kernel works also with 64 byte function alignment there is
no reason for not allowing to enforce this function alignment.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>