Add an explicit std feature so that features are purely additive.

cranelift/test-no_std.sh

@@ -21,6 +21,7 @@ for LIB in $LIBS
 do
     banner "Rust unit tests in $LIB"
     cd "lib/$LIB"
+    cargo test --no-default-features --features no_std
     cargo test --features no_std
     cd "$topdir"
 done