tests: llext: fix conflict between BUILD_ONLY and "object" case

Fix conflict between commit ce243944 ("llext: add object test case")
and commit 1408d1e5 ("tests: llext: compile architectures not
supported yet") which were tested separately but merged at the same
time.

Github "Merge Queues" can avoid this (and save resources) but:
- they're not used by Zephyr CI
- they provide confusing feedback

Signed-off-by: Marc Herbert <marc.herbert@intel.com>