amd64: do not assume that kernel is loaded at 2M physical

Allow any 2M aligned contiguous location below 4G for the staging
area location. It should still be mapped by loader at KERNBASE.

The assumption kernel makes about loader->kernel handoff with regard to
the MMU programming are explicitly listed at the beginning of hammer_time(),
where kernphys is calculated. Now kernphys is the variable instead of
symbol designating the physical address.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31121