aarch64: Add alias option support

aarch64: Add alias option support

Adds the AARCH64_OPT_EXTENSION_ALIAS macro to aarch64-option-extensions.def
to define architecture features which gate no features themselves, but
act as aliases for other features.

When getting the extension string for some architecture flags, alias features
should be used over their constituent features, even if some of the constituent
features are enabled transitively by other features.

Changes +crypto option to use this macro.

gcc/ChangeLog:

	* common/config/aarch64/aarch64-common.cc
	(struct aarch64_extension_info): Add flags_alias_preferred_over.
	(AARCH64_OPT_EXTENSION): Add setting flags preferred over to 0.
	(AARCH64_OPT_EXTENSION_ALIAS): New macro def.
	(aarch64_get_extension_string_for_isa_flags): Update to use alias
	extensions over constituent extensions.
	* config/aarch64/aarch64-feature-deps.h (alias_flags): New variable
	(AARCH64_OPT_EXTENSION): New macro def.
	(AARCH64_OPT_EXTENSION_ALIAS): New macro def.
	(HANDLE): Update to use alias_flags.
	(AARCH64_CORE): Update to use alias_flags.
	* config/aarch64/aarch64-option-extensions.def
	(AARCH64_OPT_EXTENSION_ALIAS): New macro def to
	define alias_prefer_over_flags_X.
	(crypto): Update to us AARCH64_OPT_EXTENSION_ALIAS.