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.