Only include the stubs when targeting AVR

The previous patch, e704edb, is not
enough. We cannot detect whether a downstream crate is being compiled in
test mode. #[cfg(test)] is only true when 'avr-std-stub' itself is being
tested.

Author: dylanmckay

src/lib.rs

@@ -1,10 +1,14 @@
 //! Contains simple implementations of required language items that `libstd` normally defines on
 //! other targets.
+//!
+//! You should always have `extern crate avr_std_stubs` defined when using this crate.
 
 #![no_std]
 
 #![feature(lang_items)]
 
-
-#[cfg(not(test))] // Disable the stubs for test because #[no_std] is ignored there because libtest depends on it.
+// Disable the stubs for non-AVR targets because #[no_std] is ignored
+// when using the Rust test harness, and we cannot detect if a downstream
+// crate dependent on this crate is being compiled in test mode or not.
+#[cfg(target_arch = "avr")]
 mod stubs;