crt/stdio: Fix thosand separators in printf128

This uses the same pattern with the other non-printf128 path. Here digits
are written backwards i.e. from the least significant end to the most
significant end, so they are counted as `p` is increased i.e. in humans'
order from right to left.

Signed-off-by: LIU Hao <lh_mouse@126.com>