Add new AArch64 FP16 FM{A|S} instructions.

Add new AArch64 FP16 FM{A|S} instructions.

This patch separates the new FP16 instructions backported from Armv8.4-a to Armv8.2-a
into a new flag order to distinguish them from the rest of the already existing optional
FP16 instructions in Armv8.2-a.

The new flag "+fp16fml" is available from Armv8.2-a and implies +fp16 and is mandatory on
Armv8.4-a.

gas/

	* config/tc-aarch64.c (fp16fml): New.
	* doc/c-aarch64.texi (fp16fml): New.
	* testsuite/gas/aarch64/armv8_2-a-crypto-fp16.d (fp16): Make fp16fml.
	* testsuite/gas/aarch64/armv8_3-a-crypto-fp16.d (fp16): Make fp16fml.

include/

	* opcode/aarch64.h: (AARCH64_FEATURE_F16_FML): New.
	(AARCH64_ARCH_V8_4): Enable AARCH64_FEATURE_F16_FML by default.

opcodes/

	* aarch64-tbl.h (aarch64_feature_fp_16_v8_2): Require AARCH64_FEATURE_F16_FML
	and AARCH64_FEATURE_F16.