xen: add hook for AP bootstrap memory reservation

This hook will only be implemented for bare metal, Xen doesn't require
any bootstrap code since APs are started in long mode with paging
enabled.

Approved by: gibbs
Sponsored by: Citrix Systems R&D

amd64/amd64/machdep.c:

• Set mp_bootaddress hook for bare metal.

x86/include/init.h:

• Define mp_bootaddress in init_ops.