IBM Z: stack clash prot: add missing updates of last_probe_offset

IBM Z: stack clash prot: add missing updates of last_probe_offset

After emitting probes in a loop last_probe_offset needs to be updated.
Not doing this usually assumes a too low distance to the last access
when emitting the remainder leading to stack probes being omitted.

gcc/ChangeLog:

2020-05-14  Andreas Krebbel  <krebbel@linux.ibm.com>

	* config/s390/s390.c (allocate_stack_space): Add missing updates
	of last_probe_offset.

gcc/testsuite/ChangeLog:

2020-05-14  Andreas Krebbel  <krebbel@linux.ibm.com>

	* gcc.target/s390/stack-clash-1.c: New test.