arm64: Clean up finding our load address

Use the linker to pre-calculate the offset of a known symbol from
KERNBASE, and use this to find the physical address KERNBASE should
map to.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42568

(cherry picked from commit 61f14f1da37b4e6db8f9efe40c6b544c855fb9bd)
(cherry picked from commit 257b04454818c80d9b85a840e3ea8db1ea891265)