Merge pull request #4080 from pherl/arm64

Add support for Windows ARM64 build
diff --git a/src/google/protobuf/stubs/platform_macros.h b/src/google/protobuf/stubs/platform_macros.h
index 7a54060..c3a64dd 100644
--- a/src/google/protobuf/stubs/platform_macros.h
+++ b/src/google/protobuf/stubs/platform_macros.h
@@ -50,6 +50,9 @@
 #elif defined(_M_ARM) || defined(__ARMEL__)
 #define GOOGLE_PROTOBUF_ARCH_ARM 1
 #define GOOGLE_PROTOBUF_ARCH_32_BIT 1
+#elif defined(_M_ARM64)
+#define GOOGLE_PROTOBUF_ARCH_ARM 1
+#define GOOGLE_PROTOBUF_ARCH_64_BIT 1
 #elif defined(__aarch64__)
 #define GOOGLE_PROTOBUF_ARCH_AARCH64 1
 #define GOOGLE_PROTOBUF_ARCH_64_BIT 1