kboot: Improve amd64 booting

Copy more of the necessary state for FreeBSD to boot:
o Copy EFI memory tables
o Create custom page tables needed for the kernel to find itself
o Simplify the passing of args to the trampoline by putting them

on the stack rather than in dedicated memory.

This is only partially successful... we get only part way through the
amd64 startup code before dying. However, it's much further than before
the changes.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D38259