bithelp: add count trailing zero bits variant for RISC-V
* cipher/bithelp.h (_gcry_ctz_no_zero): New. (_gcry_ctz): Use '_gcry_ctz_no_zero'. * cipher/cipher-internal.h (ocb_get_l): Use '_gcry_ctz_no_zero'.
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>