aarch64: Tweak constness of option-related data

aarch64: Tweak constness of option-related data

Some of the option structures have all-const member variables.
That doesn't seem necessary: we can just use const on the objects
that are supposed to be read-only.

Also, with the new, more C++-heavy option handling, it seems
better to use constexpr for the static data, to make sure that
we're not adding unexpected overhead.

gcc/
	* common/config/aarch64/aarch64-common.cc (aarch64_option_extension)
	(processor_name_to_arch, arch_to_arch_name): Remove const from
	member variables.
	(all_extensions, all_cores, all_architectures): Make a constexpr.
	* config/aarch64/aarch64.cc (processor): Remove const from
	member variables.
	(all_architectures): Make a constexpr.
	* config/aarch64/driver-aarch64.cc (aarch64_core_data)
	(aarch64_arch_driver_info): Remove const from member variables.
	(aarch64_cpu_data, aarch64_arches): Make a constexpr.
	(get_arch_from_id): Return a pointer to const.
	(host_detect_local_cpu): Update accordingly.