snapshot: reading from disk without BIOS!!

Both LBA and CHS coordinates are now working for the primary disk on the
primary bus.

Failure modes I ran into:
  - ATA ports are 16-bit values. Using instructions with 8-bit immediates
    will yield strange results. (I had to debug this twice because I missed
    poll-ata-primary-bus-primary-drive-regular-status-word the first time
    around.)

    Mu's toolchain has been found out here. bootstrap has good
    errors but doesn't support the instructions I need in boot.subx. The
    self-hosted phases support the instructions but provide no error-checking.
    Might be worth starting to add error-checking as I encounter the need.
    In this case, a vote for validating metadata sizes even if we don't
    validate that instructions pass in the right metadata sizes.

  - Can't poll readiness first thing. Maybe we need to always select the
    drive first.

  - Reading 8-bit values from a 16-bit port (data port 0x1f0) returns garbage.
    Reading 32-bit values however works totally fine; go figure. (Maybe
    it won't work on real hardware?)

    https://forum.osdev.org/viewtopic.php?t=36415

  - Passing in a 0 segment will never return data.

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