elfcompress: fix exit status regression in case of "Nothing to do"

elfcompress: fix exit status regression in case of "Nothing to do"

When elfcompress decides that no section data needs to be updated and
therefore the file does not have to be rewritten, it still has to exit
with a zero status indicating success.

Resolves: https://sourceware.org/bugzilla/show_bug.cgi?id=27856
Fixes: c497478390de ("elfcompress: Replace cleanup() with label")