cmd/go: skip external tests on plan9/arm

cmd/go: skip external tests on plan9/arm

CL 13166, CL 13342 and CL 33425 skipped external tests
on freebsd/arm, linux/arm and linux/mips.

This CL does the same for plan9/arm to reduce test time
on plan9/arm and prevent the Go builder to time out.

Change-Id: I16fcc5d8010a354f480673b8c4a8a11dbc833557
Reviewed-on: https://go-review.googlesource.com/90416
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>