re PR target/21981 ([4.0 only] __m64 return value should be returned in %mm0)

	PR target/21981
	* config/i386/i386.c (ix86_function_value_regno_p): Return true
	for FIRST_MMX_REG if TARGET_MMX.
	(ix86_return_in_memory): Return 1 for MMX/3dNow vectors. Delete
	wrong comment.
	(ix86_struct_value_rtx): Emit warning for MMX ABI violations.
	(ix86_value_regno): Return FIRST_MMX_REG for MMX vector modes.

From-SVN: r100832