s390/syscalls: remove system call number calculation

Explicitly write the system call number for each define instead of
calculating it. This makes it easier to parse the file when generating
system call tables for various tools and libraries.

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