x86: fix trampoline memory allocation after r332073

Add the missing breaks in the for loops, in order to exit the loop
when a suitable entry is found.

Also switch amd64 native_start_all_aps to use PHYS_TO_DMAP in order to
find the virtual address of the boot_trampoline and the initial page
tables.

Reported and tested by: pho
Sponsored by: Citrix Systems R&D