[Diffusion] rL244063: Add missing atomic libcall support.

[Dimitry Andric via cfe-commits]

dim resigned from this audit.

Users:
  jyknight (Author, Auditor)
  3.7-release (Auditor)
  cfe-commits (Auditor)
  tstellarAMD (Auditor)
  compnerd (Auditor)
  majnemer (Auditor)
  rsmith (Auditor)
  dim (Auditor)

http://reviews.llvm.org/rL244063