Rollup merge of #94415 - bjorn3:cfg_default_backend, r=Mark-Simulacrum

Use the first codegen backend in the config.toml as default

It is currently hard coded to llvm if enabled and cranelift otherwise.
This made some sense when cranelift was the only alternative codegen
backend. Since the introduction of the gcc backend this doesn't make
much sense anymore. Before this PR bootstrapping rustc using a backend
other than llvm or cranelift required changing the source of
rustc_interface. With this PR it becomes a matter of putting the right
backend as first enabled backend in config.toml.

cc ```@antoyo```

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