Fix #9928: broken printf specifiers "%La" and "%LA"

* fix unitialized variables in __printf_fphex() in case of architectures
  without support for long double - this triggered unreliable results
  or crashes when using %La or %La on x86
* activate long double implementation in use for x86_64 for x86, too,
  as they share the long double format
(cherry picked from commit d1716b277cbaf2b4265728d559b819d73330c1ef)