[PATCH] MIPS gas: Fix macro expansions for .set noat

[Eric Christopher]

> Ok, this patch converts $at uses in .set noat from warnings to errors.
> I haven't changed the .set nomacro behaviour yet, partially because
> this patch is already big enough, and partially because nomacro isn't
> fully equivalent to noat. Nomacro probably shouldn't fail if the number
> of instructions stays the same.

Agreed.

-eric