arch_loadaddr() is inconsistently used. In fact, I removed it entirely
from kboot because its interface wasn't wide enough (though I broke
powerpc kboot several years ago, and this commit might fix that). For
now, modify uboot to put it into copyin like all the other loaders
do. This might break arm ubldr.
In its place, use the new md_align() and any replacement will be called
from there, or via some explicit init() function.
These changes allow me to load a amd64 kernel via UEFI with all the
modules aligned to 16k.
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