x86: make EARLY_AP_STARTUP mandatory

[emaste]

Summary

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
Sponsored by:   The FreeBSD Foundation

[emaste]

see fdce57a04219d7a36c6646950fde6c8bcd97c044 for original commit that introduced the option

[emaste]

rG:fdce57a04219d7a36c6646950fde6c8bcd97c044

[jrtc27]

Move it from MINIMAL and GENERIC to DEFAULTS?

[wollman]

Still needs a mention in UPDATING.

[emaste]

Move it from MINIMAL and GENERIC to DEFAULTS?

D41352

Still needs a mention in UPDATING.

Indeed, although really it was needed months or years ago. @imp any suggestions on how to handle this case? Do we just add a note with today's date that mentions this was needed some time ago?

[markj]

You could also remove some hideous ifdefs from dtrace_gethrtime_init_early() for i386 and amd64. I'm happy to do that separately though.