aarch64: Enable parsing of user-provided AArch64 CPU tuning parameters

aarch64: Enable parsing of user-provided AArch64 CPU tuning parameters

This patch adds support for loading custom CPU tuning parameters from a JSON
file for AArch64 targets. The '-muser-provided-CPU=' flag accepts a user
provided JSON file and overrides the internal tuning parameters at GCC runtime.

This patch was bootstrapped and regtested on aarch64-linux-gnu, no regression.

Signed-off-by: Soumya AR <soumyaa@nvidia.com>
gcc/ChangeLog:

	* config.gcc: Add aarch64-json-tunings-parser.o.
	* config/aarch64/aarch64.cc (aarch64_override_options_internal): Invoke
	aarch64_load_tuning_params_from_json if -muser-provided-CPU= is
	(aarch64_json_tunings_tests): Extern aarch64_json_tunings_tests().
	(aarch64_run_selftests): Add aarch64_json_tunings_tests().
	* config/aarch64/aarch64.opt: New option.
	* config/aarch64/t-aarch64 (aarch64-json-tunings-parser.o): New define.
	* config/aarch64/aarch64-json-schema.h: New file.
	* config/aarch64/aarch64-json-tunings-parser.cc: New file.
	* config/aarch64/aarch64-json-tunings-parser.h: New file.