Add bits for cross-compiling to Windows using MinGW

Some tests have been updated because UTF-8 is used as the native
path encoding with MinGW/Wine, unlike MSVC/Windows.

Some of the tests fail:

- 4 Rust tests fail because long paths are not enabled and so the
  paths used when creating symlinks and junction paths are too
  long. I've tested them with x86_64-pc-windows-gnu and
  x86_64-pc-windows-gnullvm, and both see the same behaviour. The
  tests pass when the MinGW-built executable is run on Windows, so
  this is a Wine limitation.
- 12 C++ tests fail because directory symlink creation is not
  implemented. They fail whether the MinGW-built executable is run
  in Wine or on Windows, so this is a MinGW limitation.
- 1 C++ filesystem test fails because long paths are not enabled.

The failing tests are skipped at runtime when built with MinGW,
aside from the one test for long paths being enabled, which expects
them to be disabled when built with MinGW.

If long paths are enabled, e.g. by running

wine reg add HKLM\\System\\CurrentControlSet\\Control\\Filesystem /v LongPathsEnabled /t REG_DWORD /d 1 /f

then many more tests fail because the C++ tests create long paths
when that Registry value is set, but it doesn't seem to actually
enable long path support in Wine, so various filesystem operations
fail.

cpp/README.md

@@ -25,6 +25,13 @@ cmake -B build . -DCMAKE_BUILD_TYPE=RelWithDebInfo
 cmake --build build --parallel
 ```
 
+To cross-compile on Linux for Windows:
+
+```
+cmake -B build . -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_TOOLCHAIN_FILE=cmake/toolchain-mingw64.cmake
+cmake --build build --parallel
+```
+
 To build a debug build, pass `Debug` instead of `RelWithDebInfo`.
 
 The following CMake variables can be used to configure the build: