`%a` does not support setting precision (e.g. `%.6a`)

Probably best to figure out #7364 first.

Precision in `%a` format is completely ignored:
```
cargo run printf "%a %.6a\n" 0.1 0.1
0x1.999999999999ap-4 0x1.999999999999ap-4
printf "%a %.6a\n" 0.1 0.1
0xc.ccccccccccccccdp-7 0xc.cccccdp-7
```

(`format_float_hexadecimal` hardcodes a specific format)