emacs: Use stdenv.cc.libc instead of plain .libc

This is the portable way to address the system-appropriate libc,
better than conditionalizing by (darwin/GNU) system or using either
one.

pkgs/applications/editors/emacs/generic.nix

@@ -72,7 +72,7 @@ in stdenv.mkDerivation {
           "${lib.getLib libgccjit}/lib"
           "${lib.getLib libgccjit}/lib/gcc/${targetPlatform.config}/${libgccjit.version}"
           "${lib.getLib stdenv.cc.cc}/lib"
-          "${lib.getLib stdenv.libc}/lib"
+          "${lib.getLib stdenv.cc.libc}/lib"
         ]));
     in ''
       substituteInPlace lisp/emacs-lisp/comp.el --replace \