tinycc: restrict platforms to x86_64-linux

pkgs/development/compilers/tinycc/default.nix

···
     homepage = http://www.tinycc.org/;
     license = licenses.mit;
 
-    platforms = platforms.unix;
+    platforms = [ "x86_64-linux" ];
     maintainers = [ maintainers.joachifm ];
   };
 }