[AArch64] Support RAS extension for ARMv8 onwards.

[AArch64] Support RAS extension for ARMv8 onwards.

The RAS extension was introduced as part of the ARMv8.2 architecture
where it is a required feature. It is also available as an optional
feature for ARMv8 and ARMv8.1. In binutils, the RAS extension is
currently enabled by default for -march=armv8.2-a but is not available
for -march=armv8 or -march=armv8.1-a.

This patch adds the feature extension '+ras' to enable the RAS extension
for ARMv8 and ARMv8.1, it is disabled by default.

gas/
2016-04-20  Matthew Wahab  <matthew.wahab@arm.com>

	* config/tc-aarch64.c (aarch64_features): Add "ras".
	* doc/c-aarch64.texi (AArch64 Extensions): Add "ras".
	* testsuite/gas/aarch64/armv8-ras-1.d: New.
	* testsuite/gas/aarch64/armv8-ras-1.s: New.
	* testsuite/gas/aarch64/illegal-ras-1.d: New.
	* testsuite/gas/aarch64/illegal-ras-1.s: New.

Change-Id: I824fb9bc8cf846bcc03aa17a726efb1350d78b9d