Factor out architecture-common code

This commit drastically reduces duplicated code among Embedded Xinu
platforms, especially the various MIPS ports.  Platforms can now refer to
files in the directories "system/arch" or "loader/arch" to use code shared
between multiple Embedded Xinu platforms that all share the same underlying
CPU architecture.

Some ARM code has been factored out the same way, although there is
currently only one supported ARM platform (the Raspberry Pi).

Code duplication in create() was also reduced.  The platform-independent
portion is now implemented in system/create.c.  The platform-dependent
portion is delegated to a new function arch_setup_stack() that each
platform implements.

Also modified device/flash/flashInit.c:  Removed poorly designed
'flash_size' global variable that was modified directly from the loader on
one platform only, then ultimately not even used.