kboot/amd64: Use common routines for memory map parsing

Use populate_avail_from_iomem and efi_read_from_sysfs from the
refactored work rather than replicating them (imperfectly) here.

Note: memmap might need to be revisited. EFI memory maps are complex on
x86 and we might need to reconstruct it from /sys/firmware/memmap as
well as using that for the BIOS case, should we ever want to support
that again (hardware makes no sense, but many VM hosting services use
that). For now, we're going all in on EFI, though, and will revisit what
to do about BIOS later. The zfsboot project suggests BIOS support isn't
really that hard (but is a distraction atm).

Sponsored by: Netflix
Reviewed by: kevans, jhibbits
Differential Revision: https://reviews.freebsd.org/D49864