aarch64: Add support for Armv8.8-A

This patch adds skeleton support for -march=armv8.8-a, testing only
that it correctly inherits from armv8.7-a.

include/
	* opcode/aarch64.h (AARCH64_FEATURE_V8_8): New macro.
	(AARCH64_ARCH_V8_8): Likewise.

gas/
	* doc/c-aarch64.texi: Document armv8.8-a.
	* config/tc-aarch64.c (aarch64_archs): Add armv8-8-a
	* testsuite/gas/aarch64/v8-8-a.s,
	* testsuite/gas/aarch64/v8-8-a.d: New test.