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
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36675