The first word of an interface is a pointer, but for the purposes
of GC we don't need to treat it as such.
 1. If it is a non-empty interface, the pointer points to an itab
    which is always in persistentalloc space.
 2. If it is an empty interface, the pointer points to a _type.
   a. If it is a compile-time-allocated type, it points into
      the read-only data section.
   b. If it is a reflect-allocated type, it points into the Go heap.
      Reflect is responsible for keeping a reference to
      the underlying type so it won't be GCd.

If we ever have a moving GC, we need to change this for 2b (as
well as scan itabs to update their itab._type fields).

Write barriers on the first word of interfaces have already been removed.

Change-Id: I643e91d7ac4de980ac2717436eff94097c65d959
Reviewed-on: https://go-review.googlesource.com/97518
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>