cmd/compile: work with new bisect command

cmd/compile: work with new bisect command

CL 491875 introduces a new bisect command, which we plan to
document for use by end users to debug semantic changes in
the compiler and in GODEBUGs.

This CL adapts the existing GOSSAHASH support, which bisect
is a revision of, to support the specific syntax and output used
by bisect as well.

A followup CL will remove the old GOSSAHASH syntax and output
once existing consumers of that interface have been updated.

Change-Id: I99c4af54bb82c91c74bd8b8282ded968e6316f56
Reviewed-on: https://go-review.googlesource.com/c/go/+/491895
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Chase <drchase@google.com>