Like the android/linux pair, match files with GOOS=solaris if
GOOS=illumos

Follow-up for CL 174457

Fixes #32370
Updates #20603

Change-Id: Id01de8bb3990d7bbb22f735a4abed2b3cf8b4062
Reviewed-on: https://go-review.googlesource.com/c/go/+/179997
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>