CRIS: don't make asm/elf.h depend on asm/user.h

We're going to export asm/elf.h; remove its dependencies on the
non-exported asm/user.h and the unused asm/system.h include.

Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>