arm64, riscv: size boot stacks appropriately

In 8db2e8fd16c4 ("Remove the secondary_stacks array in arm64 [...]"),
bootstacks was setup to be allocated dynamically. While this is
generally how x86 does it, it inadvertently shrunk each boot stack from
KSTACK_PAGES pages to a single page.

Resize these back up to the expected size using the kstack_pages
tunable, as we'll need larger stacks with upcoming sanitizer work.

Reviewed by: andrew, imp, markj
Fixes: 8db2e8fd16c4 ("Remove the secondary_stacks array [...]")
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D36475