Lint calls to `#[no_mangle]` functions that don't have `#[inline(never)]`

[oli-obk]

Noticed when trying to debug rustc. The rust_panic function is inlined sometimes which makes it impossible to set a breakpoint on it.

If the #[no_mangle] function is just for C and never called from Rust it should be fine (though not sure how llvm copes on that and maybe still inlines it, but that seems ok, since you might even have wanted that behaviour).