rustc: fix build with unbundled jemalloc and llvm on darwin

jemalloc with stripped prefix would cause segfaults in free:
https://github.com/NixOS/nixpkgs/pull/49557#issuecomment-436734677

Thanks to @danieldk for darwin testing/debugging.

pkgs/development/libraries/jemalloc/jemalloc450.nix

@@ -1,6 +1,4 @@
-{ stdenv, fetchurl }:
 import ./common.nix {
-  inherit stdenv fetchurl;
   version = "4.5.0";
   sha256 = "10373xhpc10pgmai9fkc1z0rs029qlcb3c0qfnvkbwdlcibdh2cl";
-}
+}