libstand: Rework makefile to avoid repeated building

libstand: Rework makefile to avoid repeated building

The change to use i386 headers for x86_64 was like a sledgehammer.
From my investigation, "cpu" is not needed at all, and only four headers
are needed from "machine".  The "cpu" directory is still created so that
if some attempt to "#include <cpu/xxx>" is made, it will fail.

By adding exact header dependencies, we prevent libstand from building
over and over again in quickworld.