um: add asm/percpu.h

To make SLUB work on UML we need this_cpu_cmpxchg from
asm-generic/percpu.h.

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>