amd64 wakeup: rework trampoline page allocation

There is no need to restrict trampoline page table to low 1M, it
should work with any pages below 4G. Only wakeup code itself should
be below 1M.

Do not waste level 5 page when LA48 mode is used.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31931