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/default.nix

@@ -1,6 +1,4 @@
-{ stdenv, fetchurl, fetchpatch }:
 import ./common.nix {
-  inherit stdenv fetchurl;
   version = "5.1.0";
   sha256 = "0s3jpcyhzia8d4k0xyc67is78kg416p9yc3c2f9w6fhhqqffd5jk";
 }