lualoader: Use global printc instead of loader.printc

r330282 registered loader.printc as printc, so use it instead. This makes
sense for a couple reasons, the major point being that it reads a little bit
easier and pairs nicely with the global 'print'.

Similar cases can not really be made for other loader.* functions as most of
them are either highly specific to our use-case or usually available in
other modules, such as os. printc does not have a standard implementation
in the Lua world(*), so we have a little more leeway with it, and it's kind
of a special case of the globally available 'print'.

(*) I've been in the Lua world for all of two weeks, so this could be wrong.