IBM Z: Change Pmode to word_mode for stack probes

IBM Z: Change Pmode to word_mode for stack probes

In s390.c we are still using Pmode for the stack probes. This breaks
with -m31 -mzarch where Pmode != word_mode.

The patch also adds a new target check to s390.exp which allows us to
implement zarch specific checks in the testcases.

gcc/ChangeLog:

	* config/s390/s390.c (s390_emit_stack_probe): Change Pmode to
	word_mode.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/s390.exp: New target check s390_zarch.
	* gcc.target/s390/stack-clash-1.c: Use s390_zarch instead of lp64.
	* gcc.target/s390/stack-clash-2.c: Likewise.
	* gcc.target/s390/stack-clash-3.c: Likewise.
	* gcc.target/s390/stack-clash-5.c: New test.