include: stick to a single version (2011) C/C++ atomics

This fixes mismatched binary interface when either:
 - the versions of C and C++ were different within the LibVLC build, or
 - an out-of-tree plugin used a different language versin than LibVLC.

The LibVLC implementation for ISO C atomics with Intel intrinsics was
not generally binary compatible with the native implementation of the
compiler. In other words, only one atomics implementation can be used
in one LibVLC installation. The One Definition Rule from C++ imposes
that native implementation is used, so the LibVLC implementation is
hereby removed.

Regression from edf1c760.