Fix PR15632: No support for ppcf128 floating-point remainder on PowerPC.

For this we need to use a libcall.  Previously LLVM didn't implement
libcall support for frem, so I've added it in the usual
straightforward manner.  A test case from the bug report is included.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178639 91177308-0d34-0410-b5e6-96231b3b80d8