Make dim3 signed
Make dim3 signed

This is consistent with Numba master at present - however, in CUDA C/C++
these values are unsigned. Making them unsigned seems to cause issues in
reduce kernels, which requires investigation.

stuartarchibald/numba,sklam/numba,gmarkall/numba,numba/numba,seibert/numba,gmarkall/numba,IntelLabs/numba,cpcloud/numba,IntelLabs/numba,cpcloud/numba,sklam/numba,sklam/numba,stonebig/numba,stuartarchibald/numba,stonebig/numba,stonebig/numba,stuartarchibald/numba,seibert/numba,stuartarchibald/numba,seibert/numba,IntelLabs/numba,gmarkall/numba,IntelLabs/numba,gmarkall/numba,IntelLabs/numba,cpcloud/numba,numba/numba,numba/numba,stonebig/numba,gmarkall/numba,cpcloud/numba,cpcloud/numba,numba/numba,stuartarchibald/numba,stonebig/numba,seibert/numba,seibert/numba,sklam/numba,sklam/numba,numba/numba