Fix configuration for AIX

(backport <#3646>)
(cherry picked from commit 18dca23)

Author: Kai Luo

src/unix/mod.rs

@@ -396,7 +396,7 @@ cfg_if! {
         #[cfg_attr(feature = "rustc-dep-of-std",
                    link(name = "c", cfg(not(target_feature = "crt-static"))))]
         extern {}
-    } else if #[cfg(target_env = "aix")] {
+    } else if #[cfg(target_os = "aix")] {
         #[link(name = "c")]
         #[link(name = "m")]
         #[link(name = "bsd")]