gcc12: fix substituteInPlace on darwin

gcc-12 did a mass rename from .c to .cc c++ files. As a result build fails as:

    substitute(): ERROR: file 'gcc/config/darwin-c.c' does not exist

Closes: https://github.com/NixOS/nixpkgs/issues/172877

pkgs/development/compilers/gcc/12/default.nix

@@ -109,7 +109,7 @@ stdenv.mkDerivation ({
   # This should kill all the stdinc frameworks that gcc and friends like to
   # insert into default search paths.
   + lib.optionalString hostPlatform.isDarwin ''
-    substituteInPlace gcc/config/darwin-c.c \
+    substituteInPlace gcc/config/darwin-c.cc \
       --replace 'if (stdinc)' 'if (0)'
 
     substituteInPlace libgcc/config/t-slibgcc-darwin \