[AArch64][GAS] Add support for PAN architecture extension

[AArch64][GAS] Add support for PAN architecture extension

2015-06-01  Matthew Wahab  <matthew.wahab@arm.com>
gas/
	* config/tc-aarch64.c (parse_sys_reg): New parameter.  Check
	target support.  Fix whitespace.
	(parse_operands): Update for parse_sys_reg changes.
	(aarch64_features): Add "pan".
	* doc/c-aarch64.texi (Aarch64 Extensions): Add "pan".

gas/testsuite/
	* pan-directive.d: New.
	* pan.d: New.
	* pan.s: New

Change-Id: I161395ecea6aa607c6c7dccec797b5c558a9efad