[rustc_data_structures][perf] Simplify base_n::push_str.

This minor change removes the need to reverse resulting digits.
Since reverse is O(|digit_num|) but bounded by 128, it's unlikely
to be a noticeable in practice. At the same time, this code is
also a 1 line shorter, so combined with tiny perf win, why not?

I ran https://gist.github.com/ttsugriy/ed14860ef597ab315d4129d5f8adb191
on M1 macbook air and got a small improvement
```
Running benches/base_n_benchmark.rs (target/release/deps/base_n_benchmark-825fe5895b5c2693)
push_str/old            time:   [14.180 µs 14.313 µs 14.462 µs]
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  4 (4.00%) high mild
  1 (1.00%) high severe
push_str/new            time:   [13.741 µs 13.839 µs 13.973 µs]
                        Performance has improved.
Found 8 outliers among 100 measurements (8.00%)
  3 (3.00%) high mild
  5 (5.00%) high severe
```

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