[openmp] [cmake] Fix detecting aarch64 in standalone MSVC builds

Differential Revision: https://reviews.llvm.org/D138113

Author: mstorsjo

openmp/runtime/cmake/LibompGetArchitecture.cmake

@@ -35,7 +35,7 @@ function(libomp_get_architecture return_arch)
       #error ARCHITECTURE=arm
     #elif defined(__arm__) || defined(_M_ARM) || defined(_ARM)
       #error ARCHITECTURE=arm
-    #elif defined(__aarch64__)
+    #elif defined(__aarch64__) || defined(_M_ARM64)
       #error ARCHITECTURE=aarch64
     #elif defined(__powerpc64__) && defined(__LITTLE_ENDIAN__)
       #error ARCHITECTURE=ppc64le