cmd/internal/gc: add and test write barrier debug output

We can expand the test cases as we discover problems.
This is some basic tests plus all the things I got wrong
in some recent work.

Change-Id: Id875fcfaf74eb087ae42b441fe47a34c5b8ccb39
Reviewed-on: https://go-review.googlesource.com/9158


Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Austin Clements <austin@google.com>