cmd/dist: always use GOARM=7 for nacl/arm
cmd/dist: always use GOARM=7 for nacl/arm

LGTM=dave, rsc
R=rsc, iant, dave
CC=golang-codereviews
https://codereview.appspot.com/101590044

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