s390/purgatory: workaround llvm's IAS limitations

llvm's integrated assembler cannot handle immediate values which are
calculated with two local labels:

arch/s390/purgatory/head.S:139:11: error: invalid operand for instruction
 aghi %r8,-(.base_crash-purgatory_start)

Workaround this by partially rewriting the code.

Link: https://lore.kernel.org/r/20220511120532.2228616-6-hca@linux.ibm.com


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>