cmd/dist: skip spectralnorm shootout test on the linux-arm-arm5 builder

Temporary fix to get the arm5 builder happy again.

Without hardware floating point, this test takes over 20 minutes to
run.

A proper solution would probably be to run all the benchmark tests,
but with a much lower iteration count, just to exercise the code.

Updates golang/go#12688

Change-Id: Ie56c93d3bf2a5a693a33217ba1b1df3c6c856442
Reviewed-on: https://go-review.googlesource.com/14775


Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Minux Ma <minux@golang.org>