emacsMac: use llvm6 to build

for unclear reasons, using llvm7 crashes emacs macport. llvm6 seems to
work fine.

Fixes #61596

pkgs/top-level/all-packages.nix

@@ -17364,6 +17364,7 @@ in
     inherit (darwin.apple_sdk.frameworks)
       AppKit Carbon Cocoa IOKit OSAKit Quartz QuartzCore WebKit
       ImageCaptureCore GSS ImageIO;
+    stdenv = if stdenv.cc.isClang then llvmPackages_6.stdenv else stdenv;
   };
 
   emacsPackagesFor = emacs: self: let callPackage = newScope self; in rec {