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