Rollup merge of #36234 - durka:disable-codegen-config, r=alexcrichton

rustbuild: add config.toml option to disable codegen tests

Fixes #36232.

I think it worked? Here's a build log where I tried to bootstrap, it crashed, then I added the setting to config.toml and it continued: https://gist.github.com/durka/cbf97cf04b8e065f1a2cfda4c1b6bf95

r? @alexcrichton

diff --cc src/bootstrap/config.toml.example
Simple merge