Unfinished arm64-targeting mingw.

Notes:

Targets: windows:mingw:win64/** is a to-amd64 compiler and
accompanying prebuilt amd64 DLLs. mingw64-(cross-)gcc.aarch64.rpm is
a compiler running *on* aarch64 *producing* x86_64. The win64 project
must, under no circumstances, be changed to emit arm64 PEs, because
that would break producing amd64 PE from an aarch64 machine.

Naming: One issue with "mingw64" is that the name is ambiguous. This
project endeavour therefore uses "mingw-arm64". If the to-arm64
compiler ever materializes in full, the packages for to-x86
compilation should be renamed from mingw64-xxxx to mingw-win64-xxx /
mingw-x64-xxx or something to follow the scheme.

Issues: one complication is that clang needs to be used...