Fix parallel (NOP) host-tools.sh calls. The "build hello world" smoketest creates and deletes an output file. It used to produce this temp file in build/host, but that changed the directory timestamp so record-commands thought something changed and rebuilt its symlink list, screwing up parallel builds that were using that. The first fix moved the tempfile to host-temp, but since every host-tools build deletes and recreates that directory, parallel builds still screwed each other up. The new fix is -o /dev/null so the compile still returns an error code, but the temp file is disposed of. Multiple processes writing to /dev/null in parallel shouldn't cause a problem.