VS: Set CUDA TargetMachinePlatform explicitly on x64

The `TargetMachinePlatform` setting tells CUDA what `--machine {32,64}`
flag to pass to nvcc.  While CUDA automatically chooses the proper
default for the target architecture, it does not reflect this in the
user-visible IDE settings.  Set it explicitly to fix the user-visible
setting.

Fixes: #17355
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  External
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