timer: test arm on targets with target_has_atomic less than 64 (#1634)

tokio-timer/src/atomic.rs

@@ -7,6 +7,7 @@ pub(crate) use self::imp::AtomicU64;
 // `AtomicU64` can only be used on targets with `target_has_atomic` is 64 or greater.
 // Once `cfg_target_has_atomic` feature is stable, we can replace it with
 // `#[cfg(target_has_atomic = "64")]`.
+// Refs: https://github.com/rust-lang/rust/tree/master/src/librustc_target
 #[cfg(not(any(target_arch = "arm", target_arch = "mips", target_arch = "powerpc")))]
 mod imp {
     pub(crate) use std::sync::atomic::AtomicU64;