kboot: aarch64 bi_loadsmap

Since aarch64 is different, it needs a different smap. We first see if
we have the PA of the table from the FDT info. If so, we copy that and
quit. Otherwise, we do the best we can in translating the /proc/iomap
into EFI Memory Table format.

We also send the system table to the kernel.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D38255