Use protoc-mingw on Windows.

Native protoc-windows.exe fails to support paths longer that 1024
characters.

Needed for #276.

--
MOS_MIGRATED_REVID=113834005

third_party/BUILD

@@ -19,9 +19,10 @@ filegroup(
 filegroup(
     name = "protoc",
     srcs = select({
-        ":windows_mingw": ["protobuf/protoc-windows-x86_32.exe"],
-        ":windows_msys64_mingw64": ["protobuf/protoc-windows-x86_64.exe"],
-        ":windows_clang": ["protobuf/protoc-windows-x86_32.exe"],
+        ":windows_mingw": ["protobuf/protoc-mingw.exe"],
+        ":windows_msys64_mingw64": ["protobuf/protoc-mingw.exe"],
+        ":windows_msys64": ["protobuf/protoc-mingw.exe"],
+        ":windows_clang": ["protobuf/protoc-mingw.exe"],
         ":darwin": ["protobuf/protoc-osx-x86_32.exe"],
         ":k8": ["protobuf/protoc-linux-x86_64.exe"],
         ":piii": ["protobuf/protoc-linux-x86_32.exe"],
@@ -434,6 +435,11 @@ config_setting(
     values = {"compiler": "windows_mingw"},
 )
 
+config_setting(
+    name = "windows_msys64",
+    values = {"compiler": "windows_msys64"},
+)
+
 config_setting(
     name = "windows_msys64_mingw64",
     values = {"compiler": "windows_msys64_mingw64"},