llext: disable for Harvard ARC variants #150463

Job	Run time
twister-build-prep / prep_pr	3m 2s
twister-build-prep / prep_push	0s
twister-build (1)	26m 59s
twister-build (2)	34m 5s
twister-build (3)	27m 1s
twister-build (4)	37m 27s
twister-build (5)	25m 38s
twister-build (6)	21m 58s
twister-build (7)	28m 41s
Publish Unit Tests Results	36s
Check Twister Status / all jobs passed	1s
	3h 25m 28s

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