s390-protos.h (s390_back_chain_rtx): Add prototype.

s390-protos.h (s390_back_chain_rtx): Add prototype.

	* config/s390/s390-protos.h (s390_back_chain_rtx): Add prototype.
	* config/s390/s390.c (s390_back_chain_rtx): New function.
	* config/s390/s390.md ("allocate_stack"): Use s390_back_chain_rtx.
	Call anti_adjust_stack.
	("restore_stack_block"): Use s390_back_chain_rtx.  Enable pattern
	only if compiling with back chain.
	("save_stack_nonlocal", "restore_stack_nonlocal"): Save/restore
	back chain only if back chain enabled.  Use s390_back_chain_rtx.

From-SVN: r88087