CodeGen: Emit a libcall if the target doesn't support 16-byte wide atomics

There are targets that support i128 sized scalars but cannot emit
instructions that modify them directly.  The proper thing to do is to
emit a libcall.

This fixes PR17481.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192957 91177308-0d34-0410-b5e6-96231b3b80d8
Developer policy amendment regarding confidentiality notices

Thanks to Daniel Berlin and Nadav Rotem for feedback and rewording!

Discussion:
  http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20131014/191677.html

Reviewed by: nrotem, dberlin

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192958 91177308-0d34-0410-b5e6-96231b3b80d8