kboot: Update amd64 to use enumerate_memory_arch()

Move memory enumeration to the enumerate_memory_arch(), tweak the code a
bit to make that fit into that framework.

Also fix a bug in the name of the end location. The old code never found
memory (though amd64 doesn't yet work, this lead to using fallback
addresses that were good enough for QEMU...).

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D38250