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ARROW-3209: [C++] Rename libarrow_gpu to libarrow_cuda

Also rename the arrow::gpu namespace to arrow::cuda.

Author: Kouhei Sutou <kou@clear-code.com>
Author: Antoine Pitrou <antoine@python.org>

Closes #3088 from pitrou/ARROW-3209-rename-arrow-gpu-to-cuda and squashes the following commits:

d16eb713 <Kouhei Sutou>  Support arrow-cuda.pc in build directory
32381216 <Kouhei Sutou>  Fix path
219ffa28 <Kouhei Sutou>  Rename GPU to CUDA
f8683cac <Kouhei Sutou>  Rename GPU to CUDA
c4f7f381 <Kouhei Sutou>  Rename red-arrow-gpu to red-arrow-cuda
267de1b8 <Kouhei Sutou>  Rename libarrow-gpu-glib to libarrow-cuda-glib
9687346a <Antoine Pitrou> ARROW-3209:  Rename libarrow_gpu to libarrow_cuda