Fixes to 'Windows.EWDK.toolchain.cmake'

[MarkSchofield]

Fixes #136.

As Windows.MSVC.toolchain.cmake has been cleaned-up and improved over time it canonicalized supported CMAKE_SYSTEM_PROCESSOR values and - inadvertently broke Windows.EWDK.toolchain.cmake. Since GitHub runners don't have the EWDK installed, and don't have enough free space to install the EWDK, then it's tricky to catch such regressions. This change fixes things, making a few changes:

1. Rather than setting CMAKE_SYSTEM_PROCESSOR to $ENV{VSCMD_ARG_TGT_ARCH}, I'm checking $ENV{VSCMD_ARG_TGT_ARCH} and setting CMAKE_SYSTEM_PROCESSOR to the correct value.
2. If the $ENV{VSCMD_ARG_TGT_ARCH} corresponds to a matching CMAKE_HOST_SYSTEM_PROCESSOR, then CMAKE_SYSTEM_PROCESSOR isn't set to avoid 'cross-compiling'.
3. This change set EWDK to 1 to denote that EWDK is being used.
4. If EWDK is set, then the CUDA examples aren't compiled; they don't appear to work with the EWDK.
5. I'm adding a 'condition' to the 'ewdk' example preset so that it's excluded unless CMake is being run from an EWDK prompt.

I validated with the "EWDK_ge_release_svc_prod1_26100_250904-1728.iso" EWDK. I mounted the ISO to E: and ran:

& cmd /c "E:\BuildEnv\SetupBuildEnv.cmd x86 && set && cmake --preset ewdk && cmake --build --preset ewdk"
& cmd /c "E:\BuildEnv\SetupBuildEnv.cmd x86_arm64 && set && cmake --preset ewdk && cmake --build --preset ewdk"
& cmd /c "E:\BuildEnv\SetupBuildEnv.cmd amd64 && set && cmake --preset ewdk && cmake --build --preset ewdk"