directory before the ${MACHINE_ARCH}/${MACHINE_ARCH} directory so that
machine-specific files take precedence of architecture-specific ones.
This fixes the build on sun4v which doesn't use the sparc64 version
of mem.c.
Tested by: make universe
their own directory and module, leaving the MD parts in the MD
area (the MD parts _are_ part of the modules). /dev/mem and /dev/io
are now loadable modules, thus taking us one step further towards
a kernel created entirely out of modules. Of course, there is nothing
preventing the kernel from having these statically compiled.
