arc gas: don't leak arc_opcode_hash memory

The arc opcode hash table has entries that have a realloc'd field.
This doesn't lend itself to obstack allocation, so freeing must be
done with a purpose built hashtab del_f.

	* config/tc-arc.c (arc_opcode_free): New function.
	(md_begin): Pass the above as del_f to htab_create_alloc.
	(arc_md_end): New function.
	* config/tc-arc.h (arc_md_end): Declare.
	(md_end): Define.