Add consistent feature gating for bootstrap

[jyn514]

Currently, we have a several "best-effort" features in bootstrap, which aren't really supported, but we have code for since they're impossible to do otherwise. x.py fix, x.py clippy, and x.py test --stage 0 src/test/ui are some examples. Currently, only the last one has a feature gate: by default it gives a hard error ("this tests the beta compiler, you probably don't want that") but it lets you override that with COMPILETEST_FORCE_STAGE0=1. We should extend that to other parts of bootstrap, and use a consistent mechanism to enable the features - @Mark-Simulacrum suggested something like bootstrap-features = ["xyz"] in config.toml.

[onur-ozkan]

Implementing #112679 first would be quite beneficial. We don't have do hacky tricks to handle stage-specific bootstrap features after that.

@rustbot blocked

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