Properly gate the compiler-builtins smoke tests

Author: gnzlbg

crates/compiler-builtins-smoke-test/Cargo.toml

@@ -3,4 +3,6 @@ name = "cb"
 version = "0.1.0"
 authors = ["Jorge Aparicio <[email protected]>"]
 
-[dependencies]
+[features]
+default = []
+stable = []

crates/compiler-builtins-smoke-test/src/lib.rs

@@ -4,6 +4,10 @@
 
 #![allow(dead_code)]
 #![no_std]
+#![cfg_attr(
+    all(target_arch = "wasm32", not(feature = "stable")),
+    feature(core_intrinsics)
+)]
 
 #[path = "../../../src/math/mod.rs"]
 mod libm;