rustbuild: Clean more as part of `make clean`
rustbuild: Clean more as part of `make clean`

Clean out old documentation as well as the new test/tools directories. Should
prevent a problem that happened this morning where a PR bounced and then it left
docs with "broken links" so all future PRs bounced.

aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust