uml: create arch.h

This patch moves the declarations of the architecture hooks from user_util.h
to a new header, arch.c, and adds the necessary includes to files which need
those declarations.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>