Have Literal.Literal support the representation of NULL pointers only,
and not arbitrary pointer values.

(MachAddr <some-pointer-value-as-an-Integer>) wasn't being used,
except to handle nullAddr#. It (MachAddr) is a potential source of
problems should the compiler start doing constant folding or other
interesting operations over MachAddrs (think: interface files +
cross-compilation), so we might as well scale back the representation
of raw pointer values.