amd64: Ignore 1GB mappings in pmap_advise()

[markj]

Summary

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.

[alc]

Shouldn't the same change be made on arm64?

[markj]

In D36675#832960, @alc wrote:

Shouldn't the same change be made on arm64?

Yes, I committed it as a follow-up.