Support `#[link(wasm_import_module)]` on "Rust" ABI

Extern blocks on wasm must specify wasm_import_module for a namespace. Currently, it seems only "C"-like ABI is accepted.

#[link(wasm_import_module = "not_env")]
unsafe extern "Rust" {
    fn foo();
}

#[unsafe(no_mangle)]
extern "Rust" fn bar() {
    unsafe { foo(); }
}

The produced wasm for the above should contain

(import "not_env" "foo" ...)
(export "bar" ...)

On current stable 1.95.0, the import is namespaced to "env" instead of "not_env". On nightly, a linker error is generated instead due to the removal of --allow-undefined in rust-lang/rust#149868.

I have tested the above with "C-unwind", which works as expected. No other ABIs come to mind for wasm.

Although the ABI is unstable, which makes it nearly impossible to import a js function in a defined manner, one can satisfy the import with an export from another rust compiled wasm module with matching ABI.

Start by reproducing the provided unsafe extern "Rust" example on stable and nightly, then inspect the generated WebAssembly imports and exports. The change is done when wasm_import_module = "not_env" produces an import in the not_env namespace for the Rust ABI without the reported nightly linker error.