um: fix UML_LIB_PATH

UML_LIB_PATH is hardcoded to /usr/lib/uml/, on 64bit systems UML_LIB_PATH
needs to be /usr/lib64/uml/.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>