s390: optimize syscall function

s390: optimize syscall function

Since kernel 2.6.0 all Linux version accept the system call number
in register 1 for svc 0. There is no need to have special handling
that uses EX for system calls < 256. This will simplify and speed
up that code.

A microbenchmark doing "syscall(__NR_getpid);" in a loops gets faster
by ~12%.

        * sysdeps/unix/sysv/linux/s390/s390-32/syscall.S: Simplify
        code by always using SVC 0 instead of EX.
        * sysdeps/unix/sysv/linux/s390/s390-64/syscall.S: Likewise.