re PR target/23774 (dealloc of dynamic stack space breaks backchain)

	PR target/23774
	* config/rs6000/rs6000.md (restore_stack_block): Write the backchain
	word before changing the stack pointer.	 Use gen_frame_mem for MEMs.
	Use UNSPEC_TIE to prevent insn scheduling reordering the insns.
	(restore_stack_nonlocal): Likewise.
	(save_stack_nonlocal): Use template to emit insns, and gen_frame_mem.

From-SVN: r104206