Auto merge of #1696 - RalfJung:abi-check, r=RalfJung

add test for caller ABI check

tests/compile-fail/check_arg_abi.rs

@@ -0,0 +1,9 @@
+fn main() {
+    extern "Rust" {
+        fn malloc(size: usize) -> *mut std::ffi::c_void;
+    }
+
+    unsafe {
+        let _ = malloc(0); //~ ERROR calling a function with ABI C using caller ABI Rust
+    };
+}