[ARC] Disassemble correctly extension instructions.

[ARC] Disassemble correctly extension instructions.

For each MAJOR-MINOR opcode tuple, we can have either a 3-operand, or
2-operand, or a single operand instruction format, depending on the
values present in i-field, and a-field.

The disassembler is reading the section containing the extension
instruction format and stores them in a table.  Each table element
represents a linked list with encodings for a particular MAJOR-MINOR
tuple.

The current implementation checks only against the first element of
the list, hence, the issue.

This patch is walking the linked list until empty or finds an opcode
match.  It also adds a test outlining the found problem.

opcodes/
2016-09-15  Claudiu Zissulescu  <claziss@synopsys.com>

	* arc-dis.c (find_format): Walk the linked list pointed by einsn.

gas/
2016-09-15  Claudiu Zissulescu  <claziss@synopsys.com>

	* testsuite/gas/arc/textinsnxop.d: New file.
	* testsuite/gas/arc/textinsnxop.s: Likewise.