Enable warnings if gc is disabled

[alexcrichton]

Continuation of work in #10131. This additionally handles turning off gc-null and gc-drc and the various combinations within.

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[abrown]

I thought you wanted to fully qualify these at the use site?

[abrown]

I guess there are multiple uses in this module or something like that?

[alexcrichton]

Ideally yeah I generally prefer avoiding #[cfg] on imports since it feels pretty noisy, but this one was used so much it was getting even noisier to have fully qualified use-sites.

Probably an indicator to create crates/wasmtime/src/runtime/store/gc.rs perhaps...