x86: move EARLY_AP_STARTUP into DEFAULTS

EARLY_AP_STARTUP was introduced in 2016 (commit fdce57a04219) with note:

As a transition aid, the new behavior is moved under a new
kernel option (EARLY_AP_STARTUP). This will allow the option
to be turned off if need be during initial testing. I hope to
enable this on x86 by default in a followup commit ...

It was enabled by default, but became effectively mandatory (on x86)
some time later. Move it to DEFAULTS to avoid an unbootable system if
the option is left out of a custom kernel configuration file.

Reported by: wollman
Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41352