gccStdenvNoLibs, clangStdenvNoLibs: Init

libgcc can only be built with gcc evidentally, so we need a way for
Darwin to force gcc without bringing in the other default library
dependencies libgcc doesn't---and shouldn't---need.

pkgs/top-level/all-packages.nix

@@ -35,7 +35,7 @@ in
 
   stdenvNoCC = stdenv.override { cc = null; extraAttrs.noCC = true; };
 
-  stdenvNoLibs = let
+  mkStdenvNoLibs = stdenv: let
     bintools = stdenv.cc.bintools.override {
       libc = null;
       noLibc = true;
@@ -51,6 +51,11 @@ in
       lib.mapNullable (rs: rs ++ [ bintools ]) (stdenv.allowedRequisites or null);
   };
 
+  stdenvNoLibs = mkStdenvNoLibs stdenv;
+
+  gccStdenvNoLibs = mkStdenvNoLibs gccStdenv;
+  clangStdenvNoLibs = mkStdenvNoLibs clangStdenv;
+
   # For convenience, allow callers to get the path to Nixpkgs.
   path = ../..;