s390/ptrace: add last_break member to pt_regs

Instead of using args[0] for the value of the last breaking event
address register, add a member to make things more obvious.

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