Intrin: Add _umul128

Implement _umul128; it provides the high and low halves of a 128-bit
multiply.  We can simply use our __int128 arithmetic to implement this,
we generate great code for it:
        movq    %rdx, %rax
        mulq    %rcx
        movq    %rdx, (%r8)
        retq

Differential Revision: http://reviews.llvm.org/D6486

llvm-svn: 223175