arm64: Move the initial kernel stack out of the init_pagetables section

init_pagetables is mapped into the segment containing the BSS, but does
not get zeroed by locore. It is used for bootstrap page table pages.

It happens that the bootstrap kernel stack is also placed in that
section, but there's no reason it shouldn't live in the BSS, so move it
there. No functional change intended.

Reviewed by: andrew
MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks
Differential Revision: https://reviews.freebsd.org/D39367