x86: Fix incorrect scope of setting `shared_per_thread` [BZ# 30745]

The:

```
    if (shared_per_thread > 0 && threads > 0)
      shared_per_thread /= threads;
```

Code was accidentally moved to inside the else scope.  This doesn't
match how it was previously (before af992e7a).

This patch fixes that by putting the division after the `else` block.

(cherry picked from commit 084fb31b)