Major kernel build infrastructure changes, part 1/2 (sys).

Major kernel build infrastructure changes, part 1/2 (sys).

These changes are primarily designed to create a 2-layer machine and cpu
build hierarchy in order to support virtual kernel builds in the near term
and future porting efforts in the long term.

* Split arch/ into a set of platform architectures under machine/ and a
  set of cpu architectures under cpu/.  All platform and cpu header files
  will be accessible via <machine/*.h>.  Platform header files may override
  cpu header files (the platform header file then typically #include's the
  cpu header file).

* Any cpu header files that are not overridden will be copied directly
  into /usr/include/machine/, allowing the platform to omit those header
  files (not have to create degenerate forwarding header files).

* All source files access platform and cpu architecture files via the
  <machine/*.h> path.  The <cpu/*.h> path should only be used by
  platform header files when including the lower level cpu header files.

* Require both the 'machine' and the 'machine_arch' directives in the
  kernel config file.

* When building modules in the presence of a kernel config, use the IF
  files, use*.h files, and opt*.h files provided by the kernel config
  and do not generate them in each module's object directory.  This
  streamlines the module build considerably.