nacltest.bash: nacl/arm support. Also detect GOARCH automatically based on `uname -m`.
nacltest.bash: nacl/arm support.
Also detect GOARCH automatically based on `uname -m`.

LGTM=crawshaw, dave, rsc
R=rsc, iant, crawshaw, dave
CC=golang-codereviews
https://codereview.appspot.com/111780043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go