aarch64: Move check_required_extensions

Move SVE extension checking functionality to aarch64-builtins.cc, so
that it can be shared by non-SVE intrinsics.

gcc/ChangeLog:

	* config/aarch64/aarch64-sve-builtins.cc (check_builtin_call)
	(expand_builtin): Update calls to the below.
	(report_missing_extension, report_missing_registers)
	(check_required_extensions): Move out of aarch64_sve namespace,
	rename, and move into...
	* config/aarch64/aarch64-builtins.cc (aarch64_report_missing_extension)
	(aarch64_report_missing_registers)
	(aarch64_check_required_extensions) ...here.
	* config/aarch64/aarch64-protos.h (aarch64_check_required_extensions):
	Add prototype.