s390/syscalls: add system call table

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 <[email protected]>
Acked-by: Heiko Carstens <[email protected]>
Signed-off-by: Martin Schwidefsky <[email protected]>