[Diffusion] rL244063: Add missing atomic libcall support.

[Tom Stellard via cfe-commits]

tstellarAMD raised a concern with this commit.

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

http://reviews.llvm.org/rL244063