Crosscompilation support for same CMAKE_SYSTEM_NAME but different CMAKE_SYSTEM_PROCESSOR

Cmake won't succeed building for an arm64 CPU target on an x86_64 CPU host if both the target and the host system name is "Darwin".

This is a common use-case for macOS builds, but affects all crosscompilation targets that are built for the same CMAKE_SYSTEM_NAME.

The issue is that Modules/CMakeDetermineSystem.cmake only detects crosscompilation if CMAKE_SYSTEM_NAME is manually defined.

While this works when the target system name is differs from the host system name (e.g, building for WinCE on a linux host), this solution won't work when building for the same system name and version, but for a different target CPU architecture.

This use-case is especially common on "Darwin" machines, because it's very common to build universal binaries for macOS (binaries that can run on multiple CPU architectures).