Get timestamp with interrupt disabled

This makes `app-template` support more platforms (i.e. `thumbv6`).

src/lib.rs

@@ -16,7 +16,12 @@ fn panic() -> ! {
 }
 
 static COUNT: AtomicUsize = AtomicUsize::new(0);
-defmt::timestamp!("{=usize}", COUNT.fetch_add(1, Ordering::Relaxed));
+defmt::timestamp!("{=usize}", {
+    // NOTE(no-CAS) `timestamps` runs with interrupts disabled
+    let n = COUNT.load(Ordering::Relaxed);
+    COUNT.store(n + 1, Ordering::Relaxed);
+    n
+});
 
 /// Terminates the application and makes `probe-run` exit with exit-code = 0
 pub fn exit() -> ! {