[ARM] Add command line option for RAS extension.

[ARM] Add command line option for RAS extension.

This patch adds the architecture extension "+ras" to enable RAS
support. It is enabled by default for -march=armv8.2-a and available but
disabled by default for armv8-a and armv8.1-a.

gas/
	* config/tc-arm.c (arm_ext_v8_2): Rename to arm_ext_ras.
	(arm_ext_ras): Renamed from arm_ext_v8_2.
	(insns): Update for arm_ext_v8_2 renaming.
	(arm_extensions): Add "ras".
	* doc/c-arm.texi (ARM Options): Add an entry for "ras".
	* testsuite/gas/arm/armv8-a+ras.d: New.
	* testsuite/gas/arm/armv8_2-a.d: Add explicit command line
	options.

include/
	* opcode/arm.h (ARM_EXT2_RAS): New.  Also align preceding
	entries.
	(ARM_AEXT_V8_2A): Add ARM_EXT2_RAS.

opcodes/
	* arm-dis.c (arm_opcodes): Replace ARM_EXT_V8_2A with
	ARM_EXT_RAS in relevant entries.