kinst: be explicit about trampoline placement

The current implementation and comment was specific to amd64. Even
though in the case of kinst's supported architectures (RISC-V and ARM64)
VM_MIN_KERNEL_ADDRESS is equal to KERNBASE, it's better to be explicit.

Reviewed by: markj
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40266