Please test EARLY_AP_STARTUP

I plan to enable EARLY_AP_STARTUP on x86 in a week on HEAD.  Some folks
have been testing it for the last week or so which has exposed some 
additional things to fix.  I think I've resolved most of those in one
way or another, but it will make things smoother if other folks can
start testing this over the next few days before it is enabled by default.

(To enable, add 'options EARLY_AP_STARTUP' to your kernel config.)

Note that non-x86 platforms should eventually adopt this, but I don't
think any of them are ready yet.

-- 
John Baldwin