nds32: remove unused generic-y += cmpxchg-local.h

NDS32 is the only architecture that creates <asm/cmpxchg-local.h>,
which is not included by anyone.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>