elf-em.h: add EM_CSKY

The uapi/linux/audit.h header is going to use EM_CSKY in order
to define AUDIT_ARCH_CSKY which is needed to implement
syscall_get_arch() which in turn is required to extend
the generic ptrace API with PTRACE_GET_SYSCALL_INFO request.

The value for EM_CSKY has been taken from arch/csky/include/asm/elf.h
and confirmed by binutils:include/elf/common.h

Cc: Guo Ren <guoren@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Elvira Khabirova <lineprinter@altlinux.org>
Cc: Eugene Syromyatnikov <esyr@redhat.com>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Guo Ren <guoren@kernel.org>