Allow FFI-unsafe warnings for u128/i128

Handle new warnings on nightly, and we shouldn't need to worry about
these with compiler-builtins since this is tied to a particular compiler.

Author: alexcrichton

src/lib.rs

@@ -30,6 +30,11 @@
     )
 )]
 
+// We use `u128` in a whole bunch of places which we currently agree with the
+// compiler on ABIs and such, so we should be "good enough" for now and changes
+// to the `u128` ABI will be reflected here.
+#![allow(improper_ctypes)]
+
 // We disable #[no_mangle] for tests so that we can verify the test results
 // against the native compiler-rt implementations of the builtins.