Add some basic arm64 support

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This was tested with a raspberry pi 4 netbooted with the EDK2 UEFI pxe boot implementation. You can find prebuilt binaries for that over here: https://github.com/pftf/RPi4

Of note, this does not work with the raspberry pi eeprom's pxe implementation as best I can tell. I'm not actually convinced the eeprom pxe impl supports proxyDHCP at all since I wasn't even getting tftp or pxe traffic from it after sending the initial dhcp offer.

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For reference, I tested this against my raspberry pi 4 using https://github.com/pftf/RPi4 + pixiecore quick debian testing --arch arm64. With these code changes, the above got me to a debian net installer.