Tracking issue for cleanup of gnullvm building process

[mati865]

In #140772, targets from this family were first bootstrapped using Linux as the builder. This has some shortcomings and the upcoming releases should build natively, since there will be a proper host compiler then.

Things to clean up along with the switch:

• remove Dockerfiles for gnullvm

• rust/src/bootstrap/src/core/build_steps/dist.rs

  Line 1709 in 686bc1c

  // FIXME(mati865): `gnullvm` here is temporary, remove it once it can host itself