ci: Add a toolchain file for (cross) compilation for aarch64-w64-mingw32

ci: Add a toolchain file for (cross) compilation for aarch64-w64-mingw32

Contrary to the existing toolchain files, this doesn't hardcode
CMAKE_FIND_ROOT_PATH to a location in /usr, but locates the
compiler from the path, and picks a sysroot adjacent to it - allowing
using a compiler installed in any path.

This uses a compiler named "${TOOLCHAIN_PREFIX}-gcc" - toolchains
such as llvm-mingw do provide cross tools named "-gcc" for
compatibility, even if they use Clang.