aarch64: Adjust option parsing parameter types.

aarch64: Adjust option parsing parameter types.

Replace `const struct processor *` in output parameters with
`aarch64_arch` or `aarch64_cpu`.

Replace `std:string` parameter in aarch64_print_hint_for_extensions with
`char *`.

Also name the return parameters more clearly and consistently.

gcc/ChangeLog:

	* config/aarch64/aarch64.cc
	(aarch64_print_hint_for_extensions): Receive string as a char *.
	(aarch64_parse_arch): Don't return a const struct processor *.
	(aarch64_parse_cpu): Ditto.
	(aarch64_parse_tune): Ditto.
	(aarch64_validate_mtune): Ditto.
	(aarch64_validate_mcpu): Ditto, and use temporary variables for
	march/mcpu cross-check.
	(aarch64_validate_march): Ditto.
	(aarch64_override_options): Adjust for changed parameter types.
	(aarch64_handle_attr_arch): Ditto.
	(aarch64_handle_attr_cpu): Ditto.
	(aarch64_handle_attr_tune): Ditto.