don't test always against gcc_s

instead test half of the time against gcc_s and the other half test
against the native operation (\*).

(\*) Not all the targets have available a native version of the
intrinsics under test. On those targets we'll end up testing our
implementation against itself half of the time. This is not much of a
problem because we do several quickcheck runs per intrinsic.

library/compiler-builtins/src/lib.rs

@@ -20,6 +20,9 @@ extern crate core;
 #[cfg(test)]
 extern crate gcc_s;
 
+#[cfg(test)]
+extern crate rand;
+
 #[cfg(all(not(windows), not(target_os = "macos")))]
 extern crate rlibc;