Add missing directories to build-sequence

[dnezam]

At the moment, it appears that there are some directories that contain a Holmakefile, but are not mentioned in developers/build-sequence and hence not part of the regression suite. This lead to some directories having bit-rotted away.

The goal of this issue is to add all missing directories to build-sequence. If some directories cannot be built, they should either be fixed or removed*, potentially creating an issue if there is demand to add it back in the future.

For the future, ideally a linter would make sure that no directory can slip past the regression suite.

* Exceptions may apply.

[xrchz]

There's apparently something called check_builds in HOL4 that does the linting thing