Move Generated Symbol Files to `$BUILD_DIR`

[hohle]

Several workflows and tools will git clean or git checkout the config directory, primarily to remove splat generated files that are written to that directory. Frequently this is a destructive operation for unstaged or uncommitted changes.

Moving undefined_funcs_auto_path and undefined_syms_auto_path into $BUILD_DIR would eliminate the need for tools to manipulate the working directory as well as simplify make clean and .gitignore, and keep ephemeral files away from tracked files.