csky: Add EM_CSKY_OLD 39

C-SKY historically used 39, the same value as MCORE, from which the
architecture was derived.

C-SKY binutils support both EM_CSKY and EM_CSKY_OLD, confirmed by
binutils:include/elf/common.h

Signed-off-by: Guo Ren <ren_guo@c-sky.com>