x86: make EARLY_AP_STARTUP mandatory

When early AP startup was introduced in 2016 it was put behind a kernel
option EARLY_AP_STARTUP as a transition aid, so that it could be turned
off if necessary. For x86 the non-EARLY_AP_STARTUP case is no longer
functional, so disallow it.

Other archs are still incompatible with EARLY_AP_STARTUP, so the option
cannot yet be removed entirely.

Reported by: wollman
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41351

(cherry picked from commit 792655abd64c94146ededd991652496ec9ec0cfe)