Help stabilising mini-gmp

[Torbjörn Granlund]

There seem to be more purely arithmetic bugs in mini-gmp.  The latest is
this one:

https://gmplib.org/devel/tm/gmp/check/failure/bwldeb64v7.gmplib.org-dyn-fat:64.txt

So we might have 3 pending arithmetic bugs in mini-gmp (unless the gcd
one is a clang bug).

(This is why cleaning out 200 failures takes time, there are serious
failures lurking in their midst.)

-- 
Torbjörn
Please encrypt, key id 0xC8601622