arch: mostly remove <asm/segment.h>

A few architectures use <asm/segment.h> internally, but nothing in
common code does. Remove all the empty or almost empty versions of it,
including the asm-generic one.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>