s390: add struct tpi_info to struct pt_regs

To avoid casting ptrace members, add a union containing
both struct tpi_info and explicit int_code/int_parm members.

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