Add support for targets without atomic operations

[zeegomo]

This crate cannot currently be built for targets that do not support atomic operations (e.g. riscv32im) due to LazyBool/Usize in

getrandom/src/util.rs

Line 33 in baf2198

pub struct LazyUsize(AtomicUsize);

, even though those are not used when providing a custom implementation.

Example error when compiling for riscv32im

error[E0432]: unresolved import `core::sync::atomic::AtomicUsize`
  --> src/util.rs:12:20
   |
12 |     sync::atomic::{AtomicUsize, Ordering::Relaxed},
   |                    ^^^^^^^^^^^ no `AtomicUsize` in `sync::atomic`

It would be nice to have this working there as well, a quick workaround could be to conditionally compile those additional structs only when atomics are available.