libgcc: use forceLibgccToBuildCrtStuff

This duplicates (by reference) the two-line adjustment to libgcc's
Makefile needed in order to get crtstuff to build without a full
build of gcc.

pkgs/development/libraries/gcc/libgcc/default.nix

@@ -45,9 +45,11 @@ in stdenvNoLibs.mkDerivation (finalAttrs: {
     buildRoot=$(readlink -e "./build")
   '';
 
-  postPatch = ''
-    sourceRoot=$(readlink -e "./libgcc")
-  '';
+  postPatch =
+    gcc.cc.passthru.forceLibgccToBuildCrtStuff
+    + ''
+      sourceRoot=$(readlink -e "./libgcc")
+    '';
 
   hardeningDisable = [ "pie" ];