Add MSVC ARM64 support for reading clocks (#1052)

src/cycleclock.h

@@ -36,7 +36,7 @@
 // declarations of some other intrinsics, breaking compilation.
 // Therefore, we simply declare __rdtsc ourselves. See also
 // http://connect.microsoft.com/VisualStudio/feedback/details/262047
-#if defined(COMPILER_MSVC) && !defined(_M_IX86)
+#if defined(COMPILER_MSVC) && !defined(_M_IX86) && !defined(_M_ARM64)
 extern "C" uint64_t __rdtsc();
 #pragma intrinsic(__rdtsc)
 #endif
@@ -114,6 +114,12 @@ inline BENCHMARK_ALWAYS_INLINE int64_t Now() {
   // when I know it will work.  Otherwise, I'll use __rdtsc and hope
   // the code is being compiled with a non-ancient compiler.
   _asm rdtsc
+#elif defined(COMPILER_MSVC) && defined(_M_ARM64)
+  // See https://docs.microsoft.com/en-us/cpp/intrinsics/arm64-intrinsics?view=vs-2019
+  // and https://reviews.llvm.org/D53115
+  int64_t virtual_timer_value;
+  virtual_timer_value = _ReadStatusReg(ARM64_CNTVCT);
+  return virtual_timer_value;
 #elif defined(COMPILER_MSVC)
   return __rdtsc();
 #elif defined(BENCHMARK_OS_NACL)