Fix warnings.

Author: sunfishcode

src/backend/libc/thread/types.rs

@@ -1,4 +1,7 @@
-#[cfg(not(any(target_os = "espidf", target_os = "vita")))]
+#[cfg(all(
+    any(freebsdlike, linux_kernel, target_os = "fuchsia"),
+    not(any(target_os = "espidf", target_os = "vita"))
+))]
 use crate::backend::c;
 
 /// A command for use with [`membarrier`] and [`membarrier_cpu`].