Prior to this change, pointer maps encoded the disposition of
a word using a single bit.  A zero signaled a non-pointer
value and a one signaled a pointer value.  Interface values,
which are a effectively a union type, were conservatively
labeled as a pointer.

This change widens the logical element size of the pointer map
to two bits per word.  As before, zero signals a non-pointer
value and one signals a pointer value.  Additionally, a two
signals an iface pointer and a three signals an eface pointer.

Following other changes to the runtime, values two and three
will allow a type information to drive interpretation of the
subsequent word so only those interface values containing a
pointer value will be scanned.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12689046