kboot: Try to read UEFI memory from physical memory on aarch64

Try to open /dev/mem to read in the UEFI memory map. If we can't, then
we'll read it in the trampoline.

Retain reading in /proc/iomem to find reserved areas in Linux. We need
to know them for good places to put the kernel. These are not reflected
in the UEFI memory map. However, we should not adjust the UEFI memory
map since these reserved areas of the Linux kernel are free to be used
once we enter the kexec trampoline...

Sponsored by: Netflix
Reviewed by: tsoome, kevans, andrew
Differential Revision: https://reviews.freebsd.org/D38264