tests: Add test for sidefx handling during no-op OBJECT assignment
frontend: Fix fbcFindLibFile() to only return existing files

fbcFindLibFile() is used for the libtinfo and libgcc_eh checks which need
to know whether those libraries can be found or not. Previously they'd
always return true under the standalone build, because fbcFindLibFile()
simply built up the path to the library under lib/<target>/, without ever
checking whether the file actually existed there.