s390/runtime instrumentation: provide uapi header file

The man page for the s390_runtime_instr() system call refers to a
header file that was never exported. Therefore it's time to finally
provide the header file, and move large parts of the runtime
instrumentation header file to a new uapi header file.

Reported-by: Eugene Syromyatnikov <evgsyr@gmail.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>