arm64, riscv: Use KSTACK_PAGES for the thread0 kstack size designator

instead of kstack_pages. Although it is correct right now to use
kstack_pages on amd64 since the kern.kstack_pages tunable is not
functional on arm64, this is too fragile and wrong on riscv.

Include opt_kstack_pages.h into machdep.c to get the right definition
for KSTACK_PAGES.

Reviewed by: jhb, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D42143