asm-generic: unistd.h: fixup broken macro include.

The broken macros make the glibc compile error. If there is no
__NR3264_fstat*, we should also removed related definitions.

Reported-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Fixes: bf4b6a7d ("y2038: Remove stat64 family from default syscall set")
[arnd: Both Marcin and Guo provided this patch to fix up my clearly
       broken commit, I applied the version with the better changelog.]
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Signed-off-by: Mao Han <han_mao@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>