VS: Detect ARM64 host architecture at runtime

We use the host machine's architecture to select the `MSBuild.exe`
binary variant, and the host toolset architecture.  When CMake is
compiled as `x64` or `x86` it may still run on ARM64 hosts.  Detect the
actual architecture of the host at runtime instead of relying on the
architecture of CMake's own binary.

The `arm64/MSBuild.exe` executable is an ARM64 .NET 4 application, which
requires the ARM64 version of .NET Framework 4.8.1 to be installed on
the machine.  That version is not yet released for Windows 10; however,
the `MSBuild/Current/Bin/arm64` directory is still created when
installing Visual Studio 2022 (a user may upgrade to Windows 11 later).
Use it only if the .NET Framework is installed.

The `amd64/MSBuild.exe` executable cannot run on Windows 10 ARM64,
but can run on Windows 11 ARM64.

Fixes: #23755

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