csky: abiv1: Fixup compile error

  LD      vmlinux.o
arch/csky/lib/string.o: In function `memmove':
string.c:(.text+0x108): multiple definition of `memmove'
lib/string.o:string.c:(.text+0x7e8): first defined here
arch/csky/lib/string.o: In function `memset':
string.c:(.text+0x148): multiple definition of `memset'
lib/string.o:string.c:(.text+0x2ac): first defined here
scripts/Makefile.vmlinux_o:68: recipe for target 'vmlinux.o' failed
make[4]: *** [vmlinux.o] Error 1

Fixes: e4df2d5e

 ("csky: Add C based string functions")
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: <stable@vger.kernel.org>