Update Atomics.rst

Summary:
I changed various bits of the compilation of atomics recently, and forgot
updating the documentation. This patch just brings it up to date.

Test Plan: no change to the code

Reviewers: jfb

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5590

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218937 91177308-0d34-0410-b5e6-96231b3b80d8