Fix #13807 - foreign import nondeterminism

The problem was that the generated label included
a freshly assigned Unique value.

Test Plan:
Added a new test and looked at the generated stub:

```
 #include "HsFFI.h"
 #ifdef __cplusplus
 extern "C" {
 #endif
 extern HsInt zdmainzdAzdAzuzzlzzgzzg(StgStablePtr the_stableptr);
 extern HsInt zdmainzdAzdAzumkStringWriter(StgStablePtr the_stableptr);
 #ifdef __cplusplus
 }
 #endif
```

./validate

Reviewers: simonmar, austin, bgamari

Reviewed By: simonmar

Subscribers: rwbarton, thomie

GHC Trac Issues: #13807

Differential Revision: https://phabricator.haskell.org/D3633