Update mulredc for win64
The changes made by François Bissey to fix the "execstack" caused problems for the MinGW64 build on Windows. I have updated the m4 files to allow MinGW64 to build normally again, while also trying to keep the Bissey changes intact for other systems. This branch builds successfully on MinGW64 and on the simple Linux build environment here in gitlab.
I would be interested to hear how this branch works for other people, whether on MinGW64, Linux, x86_64, etc. Please let me know how this branch works for you, and if the changes look ok to merge. Thanks!