Fix mini_core on Windows and macOS

Author: bjorn3

example/mini_core.rs

@@ -537,6 +537,7 @@ fn panic_in_cleanup() -> ! {
 loop{}
 }
 
+#[cfg(all(unix, not(target_vendor = "apple")))]
 #[link(name = "gcc_s")]
 extern"C"{
 fn_Unwind_Resume(exc:*mut()) -> !;