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>

arch/nds32/include/asm/Kbuild

@@ -6,7 +6,6 @@ generic-y += bugs.h
 generic-y += checksum.h
 generic-y += clkdev.h
 generic-y += cmpxchg.h
-generic-y += cmpxchg-local.h
 generic-y += compat.h
 generic-y += cputime.h
 generic-y += device.h