gotest: multiple +build lines must all be true

gotest: multiple +build lines must all be true

    The code that handled +build lines did not correctly require them to all
    be true.  While looking into this I discovered that multiple +build
    lines were mishandled in a different way, because the shell does not
    preseve line breaks in backquoted data.  Look for the +build token to
    tell us when we are switching from one +build line to another.

    Reviewed-on: https://go-review.googlesource.com/25460

From-SVN: r239100