Using .mri in assembly

Using .mri in assembly

Changing mri mode between macro definition and use isn't good.  This
	.macro x
	.endm
	.mri 1
	x
leads to a segfault.  Fixed with the following patch, but I suppose
what should really happen is that macros be marked as being mri mode
when defined, and that determine whether the magic NARG parameter be
supplied at expansion.  Nobody has complained about this in 30 years
so I'm not inclined to change gas behaviour to that extent.

	* macro.c (macro_expand): Don't segfault in mri mode if NARG
	formal isn't found.