VS: Fix support for nvcc flags not in our flag table

The change in commit v3.9.0-rc4~3^2 (VS: Improve workaround for CUDA
-Xcompiler placement bug, 2017-06-21) accidentally appended to the
`AdditionalOptions` as if it were a `;`-separated list, but it is
actually a command-line string.  Append with a space instead.

While at it, fix the same problem for the `AdditionalOptions` added to
`CudaLink` by commit v3.9.0-rc3~1^2 (CUDA: When linking device code
suppress CUDA 8.0+ deprecation warnings, 2017-06-09).

Fixes: #17008