Both bazel and bazel5 need to suppress warnings about
__builtin_offsetof. For bazel, the patch for upb needs to be applied in
the top-level distdir_deps.bzl.
This also adds a workaround for
https://github.com/bazelbuild/bazel/issues/15616 which allows 'make
clean' to remove the bazel_tmp directory. This doesn't seem to be a
problem for poudriere builds for some reason but it did affect my manual
builds.