kboot: amd64 use /sys/firmware/memmap to find free memory

Use the system's firmware memory map to find a good place to put the
kernel that won't stomp on anything else. While this uses obstensibly MI
interfaces to get this data, arm64 doesn't have this, nor does
powerpc64, so place it here.

Sponsored by: Netflix

(cherry picked from commit c51e1d7c0a79a4fe2774d526ad0393cd2edd3f6c)