Now that we have architecture-specific tagging, it's useful to have a
helper function exposed that allows you to untag pointers.
This is useful for testing D100783, as we don't have to implement a tagging
helper function in userspace. As I suspect that HWASan-x64 will move to a
dynamic bit depth at runtime, exposing this helper is probably going to be
necessary in time.
Isn't this equivalent to __hwasan_tag_pointer(p, 0)?