Adds command line support for Armv8.4-A, via the new command line option -march=armv8...

Adds command line support for Armv8.4-A, via the new command line option -march=armv8.4-a.  Add support for "+dotprod" ARM feature (required for ARMv8.4-A).  Add assembler and disassembler support for new FP16 instructions introduced in Armv8.4-A

gas	* config/tc-arm.c (arm_extensions):
	(arm_archs): New entry for "armv8.4-a".
	Add FPU_ARCH_DOTPROD_NEON_VFP_ARMV8.
	(arm_ext_v8_2): New variable.
	(enum arm_reg_type): New enumeration REG_TYPE_NSD.
	(reg_expected_msgs): New entry for REG_TYPE_NSD.
	(parse_typed_reg_or_scalar): Handle REG_TYPE_NSD.
	(parse_scalar): Support REG_TYPE_VFS.
	(enum operand_parse_code): New enumerations OP_RNSD and OP_RNSD_RNSC.
	(parse_operands): Handle OP_RNSD and OP_RNSD_RNSC.
	(NEON_SHAPE_DEF): New entries for DHH and DHS.
	(neon_scalar_for_fmac_fp16_long): New function to generate Rm encoding
	for new FP16 instructions in ARMv8.2-A.
	(do_neon_fmac_maybe_scalar_long): New function to encode new FP16
	instructions in ARMv8.2-A.
	(do_neon_vfmal): Wrapper function for vfmal.
	(do_neon_vfmsl): Wrapper function for vfmsl.
	(insns): New entries for vfmal and vfmsl.
	* doc/c-arm.texi (-march): Document "armv8.4-a".
	* testsuite/gas/arm/dotprod-mandatory.d: New test.
	* testsuite/gas/arm/armv8_2-a-fp16.s: New test source.
	* testsuite/gas/arm/armv8_2-a-fp16-illegal.s: New test source.
	* testsuite/gas/arm/armv8_2-a-fp16.d: New test.
	* testsuite/gas/arm/armv8_3-a-fp16.d: New test.
	* testsuite/gas/arm/armv8_4-a-fp16.d: New test.
	* testsuite/gas/arm/armv8_2-a-fp16-thumb2.d: New test.
	* testsuite/gas/arm/armv8_2-a-fp16-illegal.d: New test.
	* testsuite/gas/arm/armv8_2-a-fp16-illegal.l: New error file.

opcodes	* arm-dis.c (coprocessor_opcodes): New entries for ARMv8.2-A new
	FP16 instructions, including vfmal.f16 and vfmsl.f16.

include	* opcode/arm.h (ARM_AEXT2_V8_4A): Include Dot Product feature.
	(ARM_EXT2_V8_4A): New macro.
	(ARM_AEXT2_V8_4A): Likewise.
	(ARM_ARCH_V8_4A): Likewise.