abuild: fix race when stripping
scanelf may pick up tempfiles created by strip or setfattr since it runs in spearate process and pipes the out to a subshell. This causes a race and may lead to the while loop attempt to strip seomthing that no longer exists. We fix that by test if file exists before try manipulate it. We could have written he file list to a temp file first, but this way we benefit from multiple cores working in parallel.