If a repo specifies `gazelle_default_attributes` to be something
different than the `gazelle_override` tag defaults,
the user should be able to pass this to the override generator
so that the output takes this into consideration, and skips
outputing redundant information.

This PR allows the user to specify a default `build_file_generation`
and `build_file_proto_mode` that is used in their repo, and generate
appropriate overrides.

These two tags need to be handled manually after the other tags because
of the
non-trivial logic in them.

**What type of PR is this?**

Bug fix

**What package or component does this PR mostly affect?**

cmd/gazelle

**What does this PR do? Why is it needed?**

This is needed to pick up the `qnx` and `osx` platforms from
bazel-contrib/rules_go@eb9a7af.

**Which issues(s) does this PR fix?**

Fixes #

**Other notes for review**

**What type of PR is this?**

Other

**What package or component does this PR mostly affect?**

dev

**What does this PR do? Why is it needed?**

**Which issues(s) does this PR fix?**

Fixes #

**Other notes for review**