AArch64: Cleanup option processing code

Further cleanup option processing. Remove the duplication of global
variables for CPU and tune settings so that CPU option processing is
simplified even further. Move global variables that need save and
restore due to target option processing into aarch64.opt. This removes
the need for explicit saving/restoring and unnecessary reparsing of
options.

gcc/
	* config/aarch64/aarch64.opt (explicit_tune_core): Rename to
	selected_tune.
	(explicit_arch): Rename to selected_arch.
	(x_aarch64_override_tune_string): Remove.
	(aarch64_ra_sign_key): Add as TargetVariable so it gets saved/restored.
	(aarch64_override_tune_string): Add Save so it gets saved/restored.
	* config/aarch64/aarch64.h (aarch64_architecture_version): Remove.
	* config/aarch64/aarch64.cc (aarch64_architecture_version): Remove.
	(processor): Remove archtecture_version field.
	(selected_arch): Remove global.
	(selected_cpu): Remove global.
	(selected_tune): Remove global.
	(aarch64_ra_sign_key): Move global to aarch64.opt so it is saved.
	(aarch64_override_options_internal): Use aarch64_get_tune_cpu.
	(aarch64_override_options): Further simplify code to only set
	selected_arch and selected_tune globals.
	(aarch64_option_save): Remove now that target options are saved.
	(aarch64_option_restore): Remove redundant target option restores.
	* config/aarch64/aarch64-c.cc (aarch64_update_cpp_builtins): Use
	AARCH64_ISA_V9.
	* config/aarch64/aarch64-opts.h (aarch64_key_type): Add, moved from...
	* config/aarch64/aarch64-protos.h (aarch64_key_type): Remove.
	(aarch64_ra_sign_key): Remove.