aarch64: Refactor check_required_extensions

Replace TARGET_GENERAL_REGS_ONLY check with an explicit check that
aarch64_isa_flags enables all required extensions.  This will be more
flexible when repurposing this function for non-SVE intrinsics.

gcc/ChangeLog:

	* config/aarch64/aarch64-sve-builtins.cc
	(check_required_registers): Remove target check and rename to...
	(report_missing_registers): ...this.
	(check_required_extensions): Refactor.