bhyve: Move AP startup code to amd64/

This code is only invoked via MD vmexit handlers. No functional change
intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40737

(cherry picked from commit c7c5d3e3888aa9018297794285dcd884e6182bd5)