x86: improve reservation of AP trampoline memory

So that it doesn't rely on physmap[1] containing an address below
1MiB. Instead scan the full physmap and search for a suitable address
to place the trampoline code (below 1MiB) and the initial memory pages
(below 4GiB).

Sponsored by: Citrix Systems R&D
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D14878