cmd/compile/internal/amd64: fix for coverage testing

Fix up a unit test to make it more friendly for coverage runs.
Currently on tip if you do

   cd ${GOROOT}/src ; go test -cover cmd/compile/...

it will cause a failure in the TestGoAMD64v1 testpoint of
cmd/compile/internal/amd64, the reason being that this testpoint
copies and reruns the test executable, expecting the rerun to produce
only the output "PASS", whereas if "-cover" is used, the output will
include percentage of statements covered as well. To fix, rework the
test to tolerate additional output if coverage is enabled.

Change-Id: I2512e06ca06e5f38108f2891ff84276d148c4f9e
Reviewed-on: https://go-review.googlesource.com/c/go/+/371234


Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>