printf: `%a` output exponent is incorrect (should be decimal, not hexadecimal)

`%a` should output "Hexadecimal floating point, lowercase"

For small numbers below 1, the exponent is printed incorrectly:
```
$ cargo run printf "%a\n" 0.5
0x1.0000000000000p+ffffffffffffffff /// This should be 0x1.0000000000000p-1
$ printf "%a\n" 0.5
0x8p-4
```

We'll cover issues with shifting (0x8 or 0x1) and padding in another issue.