Doesn't build on platforms without atomics support

[zdimension]

The crate doesn't build on platforms without atomics support, such as thumbv6m:

error[E0599]: no method named `fetch_add` found for struct `AtomicUsize` in the current scope
  --> /home/zdimension/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/url-2.5.7/src/origin.rs:68:43
   |
68 |         Self::Opaque(OpaqueOrigin(COUNTER.fetch_add(1, Ordering::SeqCst)))
   |                                           ^^^^^^^^^ method not found in `AtomicUsize`