cmake: Fix warning if a Zephyr library is created in app-mode

zephyr_append_cmake_library attempts to warn the user if a user calls it
to create a Zephyr library while the build is in application mode. At
that stage of the build, the list of Zephyr libraries has already been
processed so the function is effectively a no-op.

This heuristic used to determine if the build was in kernel mode or
application mode checked for the presence of the zephyr_prebuilt target.
This check was broken by a previous commit that renamed this target.

This commit fixes the heuristic by checking for the zephyr_pre0 target.

This was tested by calling zephyr_append_cmake_library from some
application build files, building the application, and seeing the
warning only for the call I added. No warnings were printed for any
Zephyr internal libraries.

Link: https://github.com/zephyrproject-rtos/zephyr/pull/35887
Link: https://github.com/zephyrproject-rtos/zephyr/pull/39959



Signed-off-by: Kesavan Yogeswaran <hikes@google.com>