bn/asm/*-mont.pl: harmonize with BN_from_montgomery_word.

Montgomery multiplication post-conditions in some of code paths were
formally non-constant time. Cache access pattern was result-neutral,
but a little bit asymmetric, which might have produced a signal [if
processor reordered load and stores at run-time].

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6163)

(cherry picked from commit 774ff8fe)

Resolved conflicts in ppc-mont.pl and x86_64-mont.pl.

(cherry picked from commit d067ba3b)