message: Handle macro _M_ARM64 for MSVC build
  MSVC only defines _M_ARM64 to identify Windows ARM64, so detect this macro is
  necessary to building WebRTC with MSVC.
  Cr-Commit-Position: refs/heads/master@{#29280}
diff: #define WEBRTC_ARCH_X86_64
  #define WEBRTC_ARCH_64_BITS
  #define WEBRTC_ARCH_LITTLE_ENDIAN
  -#elif defined(__aarch64__)
  +#elif defined(_M_ARM64) || defined(__aarch64__)
  #define WEBRTC_ARCH_ARM_FAMILY
  #define WEBRTC_ARCH_64_BITS
  #define WEBRTC_ARCH_LITTLE_ENDIAN
  