[ARC] Add XY registers, update neg instruction.

[ARC] Add XY registers, update neg instruction.

gas/
2016-05-23  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/tc-arc.c (md_begin): Add XY registers.
	(cpu_types): Code density is default off for ARC EM.

opcodes/
2016-05-23  Claudiu Zissulescu  <claziss@synopsys.com>

	* arc-tbl.h (neg): New instruction variant.