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`.
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