cctools: support LTO on Darwin

LTO is disabled during bootstrap to keep the bootstrap tools small and
avoid unnecessary LLVM rebuilds, but is enabled in the final stdenv
stage and should be usable by normal packages.

pkgs/os-specific/darwin/cctools/port.nix

@@ -1,5 +1,5 @@
 { stdenv, fetchFromGitHub, autoconf, automake, libtool_2, autoreconfHook
-, libcxxabi, libuuid
+, libcxxabi, libuuid, llvm
 , libobjc ? null, maloader ? null
 , enableDumpNormalizedLibArgs ? false
 }:
@@ -56,7 +56,7 @@ let
       autoreconfHook
     ];
     buildInputs = [ libuuid ] ++
-      stdenv.lib.optionals stdenv.isDarwin [ libcxxabi libobjc ];
+      stdenv.lib.optionals stdenv.isDarwin [ llvm libcxxabi libobjc ];
 
     patches = [
       ./ld-rpath-nonfinal.patch ./ld-ignore-rpath-link.patch