x86, um: get rid of arch/um/os symlink

we can get DEV_NULL defined for arch/um/drivers/null.c in less
convoluted ways, TYVM...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
5 files changed