kbuild: um fix so it compile with generic asm-offsets.h support

um has it own set of files for asm-offsets. So for now the
gen-asm-offset macro is just duplicated in the um Makefile.

This may well be the final solution since um is a bit special compared
to other architectures - time will tell.

Also added a dummy arch/um/kernel/asm-offsets.h file to keep kbuild happy.

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