bn/bn_div.c: make conditional addition unconditional

and add template for constant-time bn_div_3_words.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7589)

(cherry picked from commit 3da2e9c4)