s390/syscall: provide generic system call functions

Provide generic system call functions which should be used whenever a
system call needs to be done from user space. The only in-kernel code
is vdso, which will be converted with a follow on patch.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>