um: disable CMPXCHG_DOUBLE as it breaks UML build

Commit b789ef51 ("slub: Add cmpxchg_double_slab()") tests for
cmpxchg_double support in the SLUB code and it breaks UML builds with
SLUB.  Since UML does not support checking for CPU features, disable
CMPXCHG_DOUBLE just like CMPXCHG_LOCAL is disabled for UML.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Reviewed-by: Christoph Lameter <cl@linux.com>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
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>