libgo: don't have .lo depend on .lo.dep

libgo: don't have .lo depend on .lo.dep

    Having each .lo depend on the corresponding .lo.dep caused too many
    rebuilds, because the .lo.dep files are rebuilt when Makefile changes.
    Instead, if the .lo.dep file changes, remove the .lo file.

    Reviewed-on: https://go-review.googlesource.com/25588

From-SVN: r239258