[AArch64] ARMv8.2 command line and feature macros support

[AArch64] ARMv8.2 command line and feature macros support

	* config/aarch64/aarch64-arches.def: Add "armv8.2-a".
	* config/aarch64/aarch64.h (AARCH64_FL_V8_2): New.
	(AARCH64_FL_F16): New.
	(AARCH64_FL_FOR_ARCH8_2): New.
	(AARCH64_ISA_8_2): New.
	(AARCH64_ISA_F16): New.
	(TARGET_FP_F16INST): New.
	(TARGET_SIMD_F16INST): New.
	* config/aarch64/aarch64-option-extensions.def ("fp16"): New entry.
	("fp"): Disabling "fp" also disables "fp16".
	* config/aarch64/aarch64-c.c (arch64_update_cpp_builtins): Conditionally define
	__ARM_FEATURE_FP16_SCALAR_ARITHMETIC and __ARM_FEATURE_FP16_VECTOR_ARITHMETIC.
	* doc/invoke.texi (AArch64 Options): Document "armv8.2-a" and "fp16".

Co-Authored-By: Jiong Wang <jiong.wang@arm.com>
From-SVN: r237956