g_eli: better handling of absent/disabled CPUs

Checking hlt_cpus_mask is a no-op, and the mask will be removed in the
next commit. However, we can use the more recent CPU_ABSENT() macro to
check the status.

Reviewed by: olce
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58157

(cherry picked from commit 63d4f044225d1bb86767759b18c0ae63b25a9c03)