Fix ICE on libcall signature mismatch

[0xmuon]

Description

When code calls f32.powi(),Cranelift lowers it to a libcall named __powisf2.If you also define your own #[no_mangle] fn __powisf2() with the wrong signature, backend tried to declare the symbol twice with different types and hit an unwrap(), which caused an internal compiler error instead of a normal error message.
I fixed it by pulling the import declaration logic into a shared helper and using that for libcalls too,so signature conflicts now fail with a clear “attempt to declare __powisf2…” message like other function imports already do.

I also added a small repro in example/powi-libcall-signature.rs and a test to make sure this case errors cleanly and never ICEs again.

fixes: #1682

[bjorn3]

Thanks!