[AArch64, 2/6] Add new arch command line feaures from ARMv8.5-A

This patch is part of a series that enables ARMv8.5-A in GCC and
adds Branch Target Identification Mechanism.

This patch add all the command line feature that are added by ARMv8.5.
Optional extensions to armv8.5-a:
+rng : Random number Generation Instructions.
+memtag : Memory Tagging Extension.

ARMv8.5-A features that are optional to older arch:
+sb : Speculation barrier instruction.
+ssbs: Speculative Store Bypass Safe instruction.
+predres: Execution and Data Prediction Restriction instructions.

All of the above only effect the assembler and have already gone in the
trunk of binutils.

*** gcc/ChangeLog ***

2018-01-09  Sudakshina Das  <sudi.das@arm.com>

	* config/aarch64/aarch64-option-extensions.def: Define
	AARCH64_OPT_EXTENSION for memtag, rng, sb, ssbs and predres.
	* gcc/config/aarch64/aarch64.h (AARCH64_FL_RNG): New.
	(AARCH64_FL_MEMTAG, ARCH64_FL_SB, AARCH64_FL_SSBS): New.
	(AARCH64_FL_PREDRES): New.
	(AARCH64_FL_FOR_ARCH8_5): Add AARCH64_FL_SB, AARCH64_FL_SSBS and
	AARCH64_FL_PREDRES by default.
	* gcc/doc/invoke.texi: Document rng, memtag, sb, ssbs and predres.

From-SVN: r267766