bn/asm/ppc-mont.pl: signal no-op in 32-bit bit build.

The bug was introduced in 80d27cdb

,
one too many instructions was removed. It went unnoticed, because
new subroutine introduced in previous commit is called in real-life
RSA/DSA/DH cases, while original code is called only in rare tests.
The bug was caught in test_fuzz.

Reviewed-by: Rich Salz <rsalz@openssl.org>