Int128 support

[maleadt]

CUDA GPUs do not natively support Int128 operations. LLVM supports lowering code that works with Int128, https://reviews.llvm.org/rGb9fc48da832654a2b486adaa790ceaa6dba94455, but requires compiler intrinsics for many operations:

julia> using CUDA

julia> x = widen.(CuArray(rand(Int64, 10)))
10-element CuArray{Int128, 1}:
  ...

julia> .÷(x, x)
ERROR: LLVM error: Undefined external symbol "__divti3"

With https://reviews.llvm.org/D34708, it should be possible to resolve those intrinsics in the current module, so we can just add them to our runtime library.

[maleadt]

Alternatively, we could build compiler-rt for NVPTX and ship that in CUDA.jl like we do with libdevice.

[maleadt]

Another MWE from #793:

julia> using CUDA

julia> A = zeros(3) |> CuArray
3-element CuArray{Float64, 1}:
 0.0
 0.0
 0.0

julia> A .= UInt128(5)