amd64 pti init: fix calculation of the kernel text start

Old expression happens to provide the correct answer, but assumes that
kernel is loaded at physical address zero, with 2M gap. Do not use
kernphys to calculate KVA of kernel text start, just explicitly write
out KERNBASE and the hole size.

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