Learn `cfg`s enabled in `build.rs`

Wasmtime enables cfg(compiler) when one of its compilers (Cranelift or, soon, Winch) is available and therefore JIT compilation is available. This cfg is enabled in build.rs when the cargo features for the underlying compilers are enabled. This is much easier and more future proof than writing out cfg(any(feature = "cranelift", feature = "winch")) all over the place, which is why things are done this way.

However, rust-analyzer does not treat code blocks guarded by #[cfg(compiler)] as enabled, and fails to do autocomplete/jump-to-def/etc within these regions, even though changing the cfg to the wordier cfg(any(...)) does work.

It would be nice if rust-analyzer looked at the cfgs defined by build.rs and enabled them for the project.

Start by examining the referenced crates/wasmtime/build.rs, especially how it enables cfg(compiler) from compiler features, and reproduce the rust-analyzer behavior inside those guarded regions. Compare this with cfg(any(feature = "cranelift", feature = "winch")) and determine how support should be validated. Done means rust-analyzer recognizes build.rs-defined cfgs for autocomplete and jump-to-definition.