rustc_target: Move target env "gnu" from linux_base to linux_gnu_base

compiler/rustc_target/src/spec/mipsisa64r6_unknown_linux_gnuabi64.rs

@@ -14,7 +14,7 @@ pub fn target() -> Target {
             max_atomic_width: Some(64),
             mcount: "_mcount".to_string(),
 
-            ..super::linux_base::opts()
+            ..super::linux_gnu_base::opts()
         },
     }
 }