link_elf_obj: Colour VM objects

This will cause the VM to back sufficiently large .text sections, such
as those in zfs.ko or amdgpu.ko on amd64, with superpage mappings when
possible.

Reviewed by: alc, kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26802