kbuild: fix O=... build of um

We used include/asm-$ARCH/system.h to check if
we should create a symlink in include2 directory with
make O=... builds.
But um does not have such a file thus build filed.

Let's try anohter filename:
$ ls -d include/asm-* | wc -l
21
$ ls -d include/asm-*/errno.h | wc -l
21

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Jeff Dike <jdike@addtoit.com>