Allow a target to skip an unnecssary register return value for a struct return.

Allow a target to skip an unnecssary register return value for a struct return.

	* target.def (omit_struct_return_reg): New data hook.
	* doc/tm.texi.in: Add @hook TARGET_OMIT_STRUCT_RETURN_REG.
	* doc/tm.texi: Regenerate.
	* function.c (expand_function_end): Use it.

From-SVN: r217123