[S390] Add missing compat system call wrappers.

Add wrapper functions for the following compat system calls:
* readahead
* sendfile64
* tkill
* tgkill
* keyctl
This ensures that the high order bits of the parameter registers are correctly
sign extended.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>