s390/boot: Pass pt_regs to program check handler

Setup a pt_regs structure on the stack, poplulate it in low level assembler
code, and pass it to print_pgm_check_info(). This way there is no need to
access then lowcore from print_pgm_check_info() anymore, and the function
looks like a normal program check handler function.

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>