s390/syscalls: use generated syscall_table.h and unistd.h header files

Update the uapi/asm/unistd.h to include the generated compat and
64-bit version of the unistd.h and, as well as, the unistd_nr.h
header file.  Also remove the arch/s390/kernel/syscalls.S file
and use the generated system call table, syscall_table.h, instead.

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>