The go/build parser accepts "//+build", with no spaces.
Make the cmd/dist bootstrap parser do the same.
While in theory we should always use the space form,
I copied some code that did not into the standard tree,
and I was very confused that 'go test' had had no problem
but then make.bash died.

(As a reminder, cmd/dist does not use go/build because
cmd/dist must build against earlier versions of Go.)

Change-Id: I90a18014bd878247b8811487e5c1a7589260cbfc
Reviewed-on: https://go-review.googlesource.com/19618Reviewed-by: Ian Lance Taylor <iant@golang.org>