Generate static code for nested no-free-variable thunks

You might wonder why such things can happen, or (if they do)
why we should generate static code, but see
Note [Nested constant closures] in StgCmmBind.

I'm not quite sure why this problem has only just popped up now.
Avoid asking for non-updatable selector thunks

(Pending the fix to Trac #7771).
White space only