Now, instead of combining multiple binaries into a single BUILD_PREBUILT
definition, use separate instances for every module variant. This fixes
HOST vs HOST_CROSS prebuilts, and should be saner overall. From make,
these should look the same, we're only just using one instance of
prebuilt_internal per BUILD_PREBUILT call instead of multiple.
With that simplification, we don't have to store as much state, and can
directly write into the buffer.
Also switch from io.WriteString to fmt.Fprintln, which will require
fewer explicit string concatentations, and we don't need to worry about
newlines.
Allow the module-provided functions to return errors.
Change-Id: If30453b21fa21387f635626618d8fabfc78e6859
