gas/arc: Additional work to support multiple arc_opcode chains

gas/arc: Additional work to support multiple arc_opcode chains

Building on earlier commits, this commit moves along support for having
multiple arc_opcode entries in the arc_opcodes table that have the same
mnemonic (name) field, but are not stored in a contiguous block in the
table.

In this commit we support looking up the arc_opcode_hash_entry from the
hash table, and passing this along to the find_opcode_match function,
which then finds the specific arc_opcode that we're assembling.  We
still don't actually support the multiple chains of arc_opcode entries
in this commit, but the limitation is now isolated to the
find_opcode_match function.

There is no user visible change after this commit.

gas/ChangeLog:

	* config/tc-arc.c (arc_find_opcode): Now returns
	arc_opcode_hash_entry pointer.
	(find_opcode_match): Update argument type, extract arc_opcode from
	incoming arc_opcode_hash_entry.
	(find_special_case_pseudo): Update return type.
	(find_special_case_flag): Update return type.
	(find_special_case): Update return type.
	(assemble_tokens): Lookup arc_opcode_hash_entry based on
	instruction mnemonic, then use find_opcode_match to identify
	specific arc_opcode.