load correct syscall table for s390 and s390x

The actual syscall table for s390/s390x must be loaded depending on
the __s390x__ symbol.

building on s390 (32=bit) ends with
...
In file included from /builddir/build/BUILD/trinity-1.3/include/arch.h:27:0,
                 from tables.c:10:
tables.c: In function 'select_syscall_tables':
/builddir/build/BUILD/trinity-1.3/include/arch-s390.h:24:18: error: 'syscalls_s390x' undeclared (first use in this function)
 #define SYSCALLS syscalls_s390x
                  ^
tables.c:347:32: note: in expansion of macro 'SYSCALLS'
  syscalls = copy_syscall_table(SYSCALLS, ARRAY_SIZE(SYSCALLS));
                                ^
/builddir/build/BUILD/trinity-1.3/include/arch-s390.h:24:18: note: each undeclared identifier is reported only once for each function it appears in
 #define SYSCALLS syscalls_s390x
                  ^
tables.c:347:32: note: in expansion of macro 'SYSCALLS'
  syscalls = copy_syscall_table(SYSCALLS, ARRAY_SIZE(SYSCALLS));
                                ^
make: *** [tables.o] Error 1