Make anti_adjust_stack_and_probe_stack_clash extern and use it for Z

Make anti_adjust_stack_and_probe_stack_clash extern and use it for Z

When compiling with -mbackchain -fstack-clash-protection currently no
probes are emitted.  This patch adjusts the "allocate_stack" expander
to call anti_adjust_stack_and_probe_stack_clash when needed. In order
to do this I had to export that function from explow.c.

Ok for mainline?

gcc/ChangeLog:

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

	* config/s390/s390.md ("allocate_stack"): Call
	anti_adjust_stack_and_probe_stack_clash when stack clash
	protection is enabled.
	* explow.c (anti_adjust_stack_and_probe_stack_clash): Remove
	prototype. Remove static.
	* explow.h (anti_adjust_stack_and_probe_stack_clash): Add
	prototype.

gcc/testsuite/ChangeLog:

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

	* gcc.target/s390/stack-clash-3.c: New test.