Repair cmake libatomic check.

The cmake check for whether libatomic could be used had been
unconditionally setting the result to false. Which was somewhat
fortunate, because the prerequisite check for whether it was *needed*
was always claiming it was, even if it was not.

However, this made platforms where libatomic is actually necessary
fail to link.

Differential Revision: http://reviews.llvm.org/D10453

llvm-svn: 239819