Correct atomic libcall support for __atomic_*_fetch builtins.

In r244063, I had caused these builtins to call the same-named library
functions, __atomic_*_fetch_SIZE. However, this was incorrect: while
those functions are in fact supported by GCC's libatomic, they're not
documented by the spec (and gcc doesn't ever call them).

Instead, you're /supposed/ to call the __atomic_fetch_* builtins and
then redo the operation inline to return the final value.

Differential Revision: http://reviews.llvm.org/D14385

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@252920 91177308-0d34-0410-b5e6-96231b3b80d8