sh: Consolidate segment modifiers across mmu/nommu systems.

This moves get_fs/set_fs() and friends in to asm/segment.h. The
mm_segment_t definition is likewise consolidated from the _32/_64 split.

This is prepatory groundwork for using the generic address space limit
and verification routines across mmu/nommu configs.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>