proc: Add sysctl flag CTLFLAG_TUN to loader tunable

The sysctl variable 'kern.kstack_pages' is actually a loader tunable.
Add sysctl flag CTLFLAG_TUN to it so that sysctl -T will report it
correctly.

No functional change intended.

Note that on arm64 the thread0 stack size can not be controlled with it,
kib@ suggested that arm64 maintainers can fix it eventually so let's
enable it also on arm64 right now.

Reviewed by: kib, imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42113

(cherry picked from commit 35b3be81f1d9fe38457d9137bef7a2875d24e2c8)