Build the urbit binary on mingw-x86_64. This will build off the work done in #2. The steps required:

• Tweak third-party dependency bazel build rules in bazel/third_party/ to build on mingw-x86_64 platforms.
• Move the platform-specific third-party dependency patches in pkg/urbit/compat/mingw/ to the appropriate dependency directory in bazel/third_party/; verify the patches are still correct in the context of the dependency versions we're using; and apply the patches to the dependency builds in WORKSPACE.bazel using select().
• Migrate the platform-specific files in pkg/urbit/compat/mingw/ to pkg/noun/platform/mingw/x86_64 or pkg/vere/platform/mingw/x86_64 (whichever is more appropriate) and incorporate them in the appropriate build rule(s).
• Tweak the bazel build rules for all packages in pkg/ to build on mingw-x86_64 platforms.