Revert "gas/arc: Make .cpu directive case-insensitive"

This reverts commit 9a452709.

This change was committed as obvious, but it has been rightly been
pointed out to me that this change is not obvious, and as such I am
reverting it.

gas/ChangeLog:

	Revert prevous change.
	* config/tc-arc.c (arc_option): Make .cpu directive
	case-sensitive again.