Investigate simple performance improvement for Apple M1 about aes-prng

[mortendahl]

Note that (see https://docs.rs/aes/0.8.1/aes/#configuration-flags)

$ RUSTFLAGS="--cfg aes_armv8" cargo +nightly bench

gives much better results:

rng_fill/chacha8/2000000
                        time:   [1.5236 ms 1.5249 ms 1.5263 ms]
Found 6 outliers among 100 measurements (6.00%)
  2 (2.00%) low mild
  2 (2.00%) high mild
  2 (2.00%) high severe
rng_fill/chacha12/2000000
                        time:   [2.2282 ms 2.2300 ms 2.2320 ms]
Found 3 outliers among 100 measurements (3.00%)
  3 (3.00%) high mild
rng_fill/chacha20/2000000
                        time:   [3.6580 ms 3.6617 ms 3.6656 ms]
Found 3 outliers among 100 measurements (3.00%)
  3 (3.00%) high mild
rng_fill/aes/2000000    time:   [225.40 us 225.76 us 226.14 us]
Found 8 outliers among 100 measurements (8.00%)
  5 (5.00%) high mild
  3 (3.00%) high severe

rng_next_u64/chacha8    time:   [6.2238 us 6.2301 us 6.2366 us]
Found 4 outliers among 100 measurements (4.00%)
  1 (1.00%) low mild
  3 (3.00%) high mild
rng_next_u64/chacha12   time:   [9.0791 us 9.0910 us 9.1064 us]
Found 6 outliers among 100 measurements (6.00%)
  1 (1.00%) high mild
  5 (5.00%) high severe
rng_next_u64/chacha20   time:   [14.792 us 14.805 us 14.818 us]
Found 8 outliers among 100 measurements (8.00%)
  2 (2.00%) low mild
  4 (4.00%) high mild
  2 (2.00%) high severe
rng_next_u64/aes        time:   [1.9284 us 1.9315 us 1.9346 us]
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) low mild