llext: Rework hello_world test case to be "simple"

The simple test is there to test the API and simple extensions in
unison. Hello world was intended to be the first not the only extension
being tested.

Also refactors the entry thread to allow for usermode potentially by
passing the pointer to the function symbol rather than having it look it
up directly.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>