VS: Fix x64 host recognition by x86 cmake process
In commit 57e48f16 (VS: Add Visual Studio 16 2019 generator, 2019-01-09, v3.14.0-rc1~150^2) and commit 0fd742a6 (VS: Teach VS 2019 generator to select host tools matching host arch, 2019-01-28, v3.14.0-rc1~63^2) we intended to select the `x64` target architecture and `x64` host tools by default on x64 host machines. Fix detection of a x64 host when CMake itself is a 32-bit x86 process. The KWSys SystemInformation `Is64Bits` member is not set correctly, which led to this bug. Pending investigation on the KWSys side, simply test ourselves via `IsWow64Process`.