imgact_elf: Move GNU_ABI_VENDOR to a common header

Move the definition of GNU_ABI_VENDOR to a common location so it can
be used in multiple files.

Reviewed by: emaste, kib, imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42442

(cherry picked from commit 9d2612fc2a7e54719058c68b62ea035b029c954f)