Refactor the aarch64Create segment handling code up to the top level. Move it all into
all into seg.c, and make necessary adjustments for it being in a new file,
file, including inventing print_avail() and first_avail() to print the array
array and find the first large enough memory hole.aarch64 will use this,
and I'll refactor the other platforms to use it as I make them work.
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