llext: additional CMake harmonizations #106013

Job	Run time
twister-build-prep	3m 2s
twister-build (1)	50m 40s
twister-build (2)	45m 7s
twister-build (3)	43m 50s
twister-build (4)	36m 5s
twister-build (5)	54m 9s
twister-build (6)	23m 15s
twister-build (7)	45m 58s
twister-build (8)	50m 30s
Publish Unit Tests Results	16s
Check Twister Status / all jobs passed	3s
	5h 52m 55s

Provide feedback