fix: 64-bit atomics for platforms with no 64-bit atomics

Closes: #423

src/progress.rs

@@ -3,7 +3,7 @@ use std::{
     io::Write,
     path::Path,
     sync::{
-        atomic::{AtomicU64, AtomicU8, AtomicUsize, Ordering},
+        atomic::{AtomicU8, AtomicUsize, Ordering},
         mpsc::{self, RecvTimeoutError, Sender},
         Arc, RwLock,
     },
@@ -11,6 +11,11 @@ use std::{
     time::Duration,
 };
 
+#[cfg(not(target_has_atomic = "64"))]
+use portable_atomic::AtomicU64;
+#[cfg(target_has_atomic = "64")]
+use std::sync::atomic::AtomicU64;
+
 use crate::display::human_readable_number;
 
 /* -------------------------------------------------------------------------- */