buildall.bash: clarify target selection

Avoid confusing use of $(( in non-arithmetic context.

Permit added targets linux-386-387 linux-arm-arm5 to be correctly
matched against pattern argument.

Change-Id: Ib004c926457acb760c7e270fdd2f4095b1787a6d
Reviewed-on: https://go-review.googlesource.com/33492


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>