Fix some nits in 1G page support in the amd64 pmap.

• Move assertions out of the main loop to avoid duplicate conditional expressions, and improve assertion messages.
• Fix va_next updates. In some cases we were not doing the wraparound check before continuing the loop.
• Use the right va_next. In pmap_advise() and pmap_copy() we would step through 1G pages 2M at a time.
• Copy 1G mappings in pmap_copy().

Reviewed by: alc, kib
MFC with: r365518
Sponsored by: Juniper Networks, Inc., Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D26463