This adds SDL and DD 64-bit builds to Appveyor. I use x86_64 GCC 7.3.0, located in C:\mingw-w64\x86_64-7.3.0-posix-seh-rt_v5-rev0\mingw64.
I also disable UPX by default, because we're not using that for release builds due to virus scan issues. The user can override this by setting NOUPX=0 in Appveyor's project settings.
This requires !440 (merged) to be merged because that MR handles MINGW64 earlier in the Makefile, which avoids a compile error.
Results and artifacts: https://ci.appveyor.com/project/mazmazz/srb2/builds/21290671