aarch64: Add command-line support for Armv8.7-a

aarch64: Add command-line support for Armv8.7-a

This patch adds support for -march=armv8.7-a in GCC.
It adds the +ls64 extension that's included in this architecture revision.
Currently this is just the command-line option and +ls64 allows the relevant instructions
to be used in inline assembly. The ACLE defines some intrinsics for them but those can be
added separately later (together with the appropriate __ARM_FEATURE_* predefine).

2021-09-28  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

	* config/aarch64/aarch64.h (AARCH64_FL_LS64): Define
	(AARCH64_FL_V8_7): Likewise.
	(AARCH64_FL_FOR_ARCH8_7): Likewise.
	* config/aarch64/aarch64-arches.def (armv8.7-a): Define.
	* config/aarch64/aarch64-option-extensions.def (ls64): Define.
	* doc/invoke.texi: Document the above.