ARM64EC target defines x86_64 macros to ensure that x86_64-specific code paths are used, but for code that really needs to use aarch64 branch there is an additional _M_ARM64EC macro that needs to be used. See https://techcommunity.microsoft.com/t5/windows-os-platform-blog/getting-to-k... for details.
-- v2: include: Support ARM64EC target in winnt.h. include: Move __getReg intrinsic to intrin.h. include: Support ARM64EC target in intrin.h.