gas/arc: Make member of arc_flags const

gas/arc: Make member of arc_flags const

By making the flgp field of struct arc_flags constant we can remove a
place where we cast away the const-ness of a variable.  Also, given that
the value assigned to this field almost always comes from compile-time
constant data, having the field non-constant is probably a bad thing.

gas/ChangeLog:

	* config/tc-arc.c (find_opcode_match): Remove casting away of
	const.
	* config/tc-arc.h (struct arc_flags): Make flgp field const.