llvm: fix broken llvm-config-native for canExecute

Since a0b4b85bfa ("llvm: Avoid cross compiling if the build platform
can execute host binaries"), the flags to get a working
llvm-config-native are not used when the build platform can execute
host binaries, resulting in a broken llvm-config-native, and therefore
a broken mesa, but since build can execute host, we don't need a
separate llvm-config-native at all — we can just use the normal
llvm-config.

Fixes: a0b4b85bfa ("llvm: Avoid cross compiling if the build platform can execute host binaries")

pkgs/development/compilers/llvm/common/llvm/default.nix

@@ -417,9 +417,11 @@ stdenv.mkDerivation (rec {
   + optionalString (stdenv.isDarwin && enableSharedLibraries) ''
     ln -s $lib/lib/libLLVM.dylib $lib/lib/libLLVM-${release_version}.dylib
   ''
-  + optionalString (stdenv.buildPlatform != stdenv.hostPlatform) ''
+  + optionalString (stdenv.buildPlatform != stdenv.hostPlatform) (if stdenv.buildPlatform.canExecute stdenv.hostPlatform then ''
+    ln -s $dev/bin/llvm-config $dev/bin/llvm-config-native
+  '' else ''
     cp NATIVE/bin/llvm-config $dev/bin/llvm-config-native
-  '';
+  '');
 
   inherit doCheck;