s390/syscalls: simplify syscall_get_arch()

Given that sizeof(long) is now always 8, we can simplify syscall_get_arch()
a bit.
Just another piece I didn't find when removing 31 bit support.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>