aarch64: Add basic support for armv8.7-a architecture

aarch64: Add basic support for armv8.7-a architecture

This patch adds support for AArch64 -march=armv8.7-a command line option
in GAS.

Please note that this change ONLY extends -march= command line interface
with a new "armv8.7-a" option. Architectural changes like new instructions
will be added in following patches.

gas/ChangeLog:

2020-10-16  Przemyslaw Wirkus  <przemyslaw.wirkus@arm.com>

	* NEWS: Docs update.
	* config/tc-aarch64.c (armv8.7-a): New arch.
	* doc/c-aarch64.texi (-march=armv8.7-a): Update docs.

include/ChangeLog:

2020-10-16  Przemyslaw Wirkus  <przemyslaw.wirkus@arm.com>

	* opcode/aarch64.h (AARCH64_FEATURE_V8_7): New feature bitmask.
	(AARCH64_ARCH_V8_7): New arch feature set.

opcodes/ChangeLog:

2020-10-16  Przemyslaw Wirkus  <przemyslaw.wirkus@arm.com>

	* aarch64-tbl.h (ARMV8_7): New macro.