This allows us to see the effect of any specicalized implementations for
`getrandom::u32` or `getrandom::u64`. As expected, on Linux (which just
uses the default implementation in `utils.rs`) there is no change:
```
test bench_u32                 ... bench:         196.50 ns/iter (+/- 4.85) = 20 MB/s
test bench_u32_via_fill        ... bench:         198.25 ns/iter (+/- 1.78) = 20 MB/s
test bench_u64                 ... bench:         196.95 ns/iter (+/- 2.99) = 40 MB/s
test bench_u64_via_fill        ... bench:         197.62 ns/iter (+/- 2.24) = 40 MB/s
```
but when using the `rdrand` backend (which is specialized), there is a
mesurable difference.
```
test bench_u32                 ... bench:          16.84 ns/iter (+/- 0.09) = 250 MB/s
test bench_u32_via_fill        ... bench:          18.40 ns/iter (+/- 0.28) = 222 MB/s
test bench_u64                 ... bench:          16.62 ns/iter (+/- 0.06) = 500 MB/s
test bench_u64_via_fill        ... bench:          17.70 ns/iter (+/- 0.08) = 470 MB/s
```

### Fixed
- Build error on Android [#588]

[#588]: #588