amd64: Ignore 1GB mappings in pmap_advise()

This assertion can be triggered by usermode since vm_map_madvise()
doesn't force advice to be applied to an entire largepage mapping. I
can't see any reason not to permit it, however, since MADV_DONTNEED and
_FREE are advisory and we can simply do nothing when a 1GB mapping is
encountered.

Reviewed by: alc, kib

(cherry picked from commit 4551cbbe9906812556501c541fef0d88b0398b27)