CMakeSystem: include toolchain file after setting CMAKE_HOST_ (#13796)

Make the CMAKE_HOST_* variables always available in the toolchain file.
They are when the file is included in CMakeDetermineSystem.cmake, and
now also here.  For the non-cross-compiling case this does not change
anything at all.

Alex
Merge topic 'CrossCompilingFix13796' into next

efb19b6 CMakeSystem: include toolchain file after setting CMAKE_HOST_ (#13796)