Add a new SYSTEM_ONLY mode for CMAKE_FIND_ROOT_PATH.

This new SYSTEM_ONLY mode allows for the intrinsic system paths
used for find_* commands to get re-rooted from the CMAKE_FIND_ROOT_PATH
variable while maintaining original user-specified paths.  The intent is
for this to be the preferred mode for cross compiling.  It allows for a
a restricted default search within a desired system root but also
lets the user specify explicit search paths anythere on the host system.
Merge topic 'add-find-root-system_only-mode' into next

2e08cf8e Add a new SYSTEM_ONLY mode for CMAKE_FIND_ROOT_PATH.