Fix #[macro_use] extern crate in sys/cloudabi

Author: taiki-e

src/libstd/sys/cloudabi/abi/bitflags.rs

@@ -25,8 +25,7 @@
 // ignore-license
 
 #[cfg(feature = "bitflags")]
-#[macro_use]
-extern crate bitflags;
+use bitflags::bitflags;
 
 // Minimal implementation of bitflags! in case we can't depend on the bitflags
 // crate. Only implements `bits()` and a `from_bits_truncate()` that doesn't