uml: add asm-um/asm.h

git-x86.patch introduces asm-x86/asm.h.  UML needs to follow this with
an asm/asm.h of its own.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>