GPU: make std::is_pointer address-space aware on Metal

[ktf]

The shim's partial specialization on a bare T* does match a pointer type written
out in full, but not one deduced from an argument, which carries its address
space. RDHUtils uses std::is_pointer to keep its pointer overloads apart from
its reference ones, so the reference template was instantiated for a pointer and
dereferenced it as a struct.

metal::is_pointer is address-space aware, so the Metal branch forwards to it.