Fix signed/unsigned mismatch warnings, part 8.
Fix signed/unsigned mismatch warnings, part 8.

These f2s.c changes don't have any d2s.c counterparts.

Relevant signatures:

```
int32_t pow5bits(int32_t e);
uint32_t mulPow5InvDivPow2(uint32_t m, uint32_t q, int32_t j);
uint32_t mulPow5divPow2(uint32_t m, uint32_t i, int32_t j);
```

ulfjack/ryu,ulfjack/ryu,ulfjack/ryu,ulfjack/ryu