gas/arc: Modify structure used to hold opcodes

gas/arc: Modify structure used to hold opcodes

The arc assembler builds a hash table to hold references to arc_opcode
entries in the arc_opcodes table.  This hash assumes that each mnemonic
will always appear in a contiguous blocks within the arc_opcodes table;
all ADD instruction will be together, all AND instructions will likewise
be together and so on.

The problem with this is that as different variations of arc are added,
then it is often more convenient to split instructions apart, so all the
base ADD instructions are together, but, variants of ADD specific to one
variation of arc are grouped with other instructions specific to that
arc variant.  The current data structures don't support splitting the
instructions in this way.

This commit is a first step towards addressing this limitation.  In this
commit the hash table that currently holds arc_opcode pointers directly,
instead holds a pointer to a new, intermediate, data structure.  This
new data structure holds the pointer to the arc_opcode.  In this way, we
can, in the future support having the intermediate structure hold
multiple pointers to different arc_opcode groups.

There should be no visible functional change after this commit.

gas/ChangeLog:

	* config/tc-arc.c (struct arc_opcode_hash_entry): New structure.
	(arc_find_opcode): New function.
	(find_special_case_pseudo): Use arc_find_opcode.
	(find_special_case_flag): Likewise.
	(assemble_tokens): Likewise.
	(md_begin): Build hash using struct arc_opcode_hash_entry.