llext: add reference counting

Extend the llext_load() / llext_unload() API to let it be called
repeatedly for the same extension to increment or decrement its
reference counter respectively. We use a mutex to protect the counter
and make both llext_load() and llext_unload() return the use-count to
let the caller identify when the first loading and the last unloading
took place.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>