s390/syscalls: add system call table

Add a table that lists available system calls and their entry
functions.  With this change, adding new system calls become
fairly easy:

  1. Add a new table entry consisting of the system call number,
     abi (common or 64/31 bit only), and the entry functions.
  2. If necessary, create a compat syscall wrapper.
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>