make SwitchToThread only callable from std

Author: beepster4096

src/shims/windows/foreign_items.rs

@@ -372,7 +372,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 // FIXME: we should set last_error, but to what?
                 this.write_null(dest)?;
             }
-            "SwitchToThread" => {
+            // this is only callable from std because we know that std ignores the return value
+            "SwitchToThread" if this.frame_in_std() => {
                 let [] = this.check_shim(abi, Abi::System { unwind: false }, link_name, args)?;
 
                 this.yield_active_thread();