validate_target_files: support gki_arm64

gki_arm64 is a special target to build multiple GKI boot-*.img
files. It has neither system.img nor vbmeta.img. Skips checking
system.img and vbmeta.img in this case.

Bug: 223078405
Test: validate_target_files signed-gki_arm64-target_files-8277156.zip
Change-Id: I9d3c102c11787cf70c1e1fc8e6a552f8fd19ff72