target-insns.def: Add memory_blockage.

target-insns.def: Add memory_blockage.

	* target-insns.def: Add memory_blockage.
	* optabs.c (expand_memory_blockage): New function.
	(expand_asm_memory_barrier): Rename ...
	(expand_asm_memory_blockage): ... to this.
	(expand_mem_thread_fence): Call expand_memory_blockage
	instead of expand_asm_memory_barrier.
	(expand_mem_singnal_fence): Ditto.
	(expand_atomic_load): Ditto.
	(expand_atomic_store): Ditto.
	* doc/md.texi (Standard Pattern Names For Generation):
	Document memory_blockage instruction pattern.

From-SVN: r253750