Changes for GMP 6.1.2

[Torbjörn Granlund]

I've now committed all relevant non mini-gmp changes to the gmp-6.1
repo.

When we're ready with the mini-gmp merges, let's throw the test machines
on the gmp-6.1 repo instead of the head repo.  A test cycle is 6 days
(without manual intervention), so if things work smoothly, we should be
able to release well before the Yule period.

-- 
Torbjörn
Please encrypt, key id 0xC8601622