Add missing atomic libcall support.

Support for emitting libcalls for __atomic_fetch_nand and
__atomic_{add,sub,and,or,xor,nand}_fetch was missing; add it, and some
test cases.

Differential Revision: http://reviews.llvm.org/D10847

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@244063 91177308-0d34-0410-b5e6-96231b3b80d8