s390/syscalls: move __S390_SYS_STUBx() macro

Move __S390_SYS_STUBx() the end of the CONFIG_COMPAT section, so both
variants (compat and non-compat) are close together and can be easily
compared.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>