x2APIC mode is detected and enabled. Current theory is that switching
the APIC mode while an IPI is in flight might be the issue.
Postpone switching to x2APIC mode until we are guaranteed that all
starting IPIs are already send and aknowledged. Use aps_ready signal
as an indication that the BSP is done with us.
Tested by: adrian
Sponsored by: The FreeBSD Foundation
MFC after: 2 months