s390/syscalls: add syscalltbl script

Introduce the syscalltbl script to read the system call table and
generate respective uapi unistd.h header files.  Also it generates
the contents syscalls.S - the real system call table - which is
included by arch/s390/kernel/entry.S.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>