sparc: Add -mmemory-model command-line option.

	* config/sparc/sparc-opts.h (enum sparc_memory_model_type): New.
	* config/sparc/sparc.opt (mmemory-model=): New option.
	* doc/invoke.texi (Sparc Options): Document it.
	* config/sparc/sparc.c (sparc_option_override): Provide default
	for sparc_memory_model.
	(sparc_emit_membar_for_model): Omit barrier combinations that are
	implied by the memory model.

From-SVN: r181853