I've reviewed masamoto's last patch 3.5-issue21085-struct_siginfo-2.patch

It applies cleanly and allows the signals module to compile on Cygwin64 2.5.1 / Windows 10.  I attached versions of the patch that apply cleanly to tip, 3.4, and 3.3 as well.

However, even with this issue resolved it can't be fully tested because there are other open issues preventing a successful build on Cygwin, including issue25658 and issue13756

Would it be ok to commit this even though it doesn't resolve all build issues?  Or should I try to cobble together a meta-issue of current known build issues and make sure they all have working patches first?