aarch64: Add base support for Armv8-R

aarch64: Add base support for Armv8-R

This patch adds the basic infrastructure needed to support Armv8-R in
AArch64 binutils: new command-line flags, new feature bits, a new BFD
architecture, and support for differentiating between architecture
variants in the disassembler.

The new command-line options added by this patch are -march=armv8-r in
GAS and -m aarch64:armv8-r in objdump.

The disassembler support is necessary since Armv8-R AArch64 introduces a
system register (VSCTLR_EL2) which shares an encoding with a different
system register (TTBR0_EL2) in Armv8-A. This also allows us to use the
correct preferred disassembly for the new DFB alias introduced in
Armv8-R.

bfd/ChangeLog:

2020-09-08  Alex Coplan  <alex.coplan@arm.com>

	* archures.c (bfd_mach_aarch64_8R): New.
	* bfd-in2.h: Regenerate.
	* cpu-aarch64.c (bfd_aarch64_arch_v8_r): New.
	(bfd_aarch64_arch_ilp32): Update tail pointer.

gas/ChangeLog:

2020-09-08  Alex Coplan  <alex.coplan@arm.com>

	* config/tc-aarch64.c (aarch64_archs): Add armv8-r.
	* doc/c-aarch64.texi: Document -march=armv8-r.

include/ChangeLog:

2020-09-08  Alex Coplan  <alex.coplan@arm.com>

	* opcode/aarch64.h (AARCH64_FEATURE_V8_A): New.
	(AARCH64_FEATURE_V8_R): New.
	(AARCH64_ARCH_V8): Include new A-profile feature bit.
	(AARCH64_ARCH_V8_R): New.

opcodes/ChangeLog:

2020-09-08  Alex Coplan  <alex.coplan@arm.com>

	* aarch64-dis.c (arch_variant): New.
	(determine_disassembling_preference): Disassemble according to
	arch variant.
	(select_aarch64_variant): New.
	(print_insn_aarch64): Set feature set.