kbuild: cris use generic asm-offsets.h support

Cris has a dedicated asm-offsets.c file per subarchitecture.
So a symlink is created to put the desired asm-offsets.c file
in $(ARCH)/kernel
This is absolutely not good practice, but it was the trick
used in the rest of the cris code.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>