amd64: Make start_all_aps() static

It is not used elsewhere since the change [1].

[1] ac3ede5371af x86/xen: remove PVHv1 code

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D55668

(cherry picked from commit 5f0ab9d9e965225c4af0c6ed481e01eee0ffab8f)
(cherry picked from commit f521487cd935a0a547a9d7b54eeadebb200f5148)