CheckerGccPlugins: Updates.

Updates to thread checker flag file logic: When searching for the ATLAS_CHECK_THREAD_SAFETY flag file for the thread checker, stop walking up the directory tree when we hit the root of a package, indicated by the presence of CMakeLists.txt. Also look in an include directory in that case. Some other tweaks to keep the search from uselessly going up the tree for release builds and some external packages.

Ignore unchecked root calls: Don't warn about calls to unchecked root functions. (Won't have any effect in the release build anyway, since unchecked call warnings are disabled.) Print out the declaration locus for unchecked calls.

Add FileStagerAlg to exclusion list of gaudi inheritance checker. FileStagerAlg has minimial dependencies (and not AthenaBaseComps). Also add TestWhiteBoard, used for unit testing in AthenaKernel.

Fix uninitialized variable: In gcc62, the member opt_pass::graph_dump_initialized is left uninitialized, which leads to ubsan warnings. Explicitly initialize the member to avoid this. (This was fixed as of gcc7.)