logging: Assign IDs to log backends early during log_core_init

This commit moves the assignment of backend IDs from the 'z_log_init'
function to the earlier 'log_core_init' function. This ensures that
backend IDs are assigned before they are used in the 'log_backend_enable'
function, preventing incorrect settings of log dynamic filters.

Signed-off-by: Dong Wang <dong.d.wang@intel.com>