EWDK toolchain is broken - it uses 'old' constructs

[MarkSchofield]

When the EWDK toolchain was added, the Windows.MSVC.toolchain.cmake file (which it wraps) was inconsistent with its use of CMAKE_SYSTEM_PROCESSOR, allowing, say, x86 and x64 (which should really be X86 and AMD64). Windows.MSVC.toolchain.cmake is now more strict - removing support for the older values. But the EWDK toolchain was dependent on those values, and is broken now that they have been removed. Sadly, there's no CI builds for EWDK builds, since the EWDK isn't installed on Agents by default, and there's not enough space on the Agents to download the EWDK.

The Windows.EWDK.toolchain.cmake file should explicitly map the EWDK environment variables to the supported Windows.MSVC.toolchain.cmake constructs.