aarch64: Add support for Armv8-R DFB alias

aarch64: Add support for Armv8-R DFB alias

This adds support for the DFB alias introduced in Armv8-R AArch64.

gas/ChangeLog:

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

	* testsuite/gas/aarch64/dfb.d: New test.
	* testsuite/gas/aarch64/dfb.s: Input.

opcodes/ChangeLog:

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

	* aarch64-tbl.h (aarch64_feature_v8_r): New.
	(ARMV8_R): New.
	(V8_R_INSN): New.
	(aarch64_opcode_table): Add dfb.
	* aarch64-opc-2.c: Regenerate.
	* aarch64-asm-2.c: Regenerate.
	* aarch64-dis-2.c: Regenerate.