rs6000: Emit ROP-mitigation instructions in prologue and epilogue

rs6000: Emit ROP-mitigation instructions in prologue and epilogue

2021-05-14  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	* config/rs6000/rs6000-internal.h (rs6000_stack): Add
	rop_hash_save_offset and rop_hash_size.
	* config/rs6000/rs6000-logue.c (rs6000_stack_info): Compute
	rop_hash_size and rop_hash_save_offset.
	(debug_stack_info): Dump rop_hash_save_offset and rop_hash_size.
	(rs6000_emit_prologue): Emit hashst[p] in prologue.
	(rs6000_emit_epilogue): Emit hashchk[p] in epilogue.
	* config/rs6000/rs6000.md (unspec): Add UNSPEC_HASHST and
	UNSPEC_HASHCHK.
	(hashst): New define_insn.
	(hashchk): Likewise.