[Aarch64] Fix assumption that Windows implies x86

When compiling for Windows on Arm the amd64 debug interfce from the Visual
Studio SDK is used as the cmake currently only distinguishes between x86 and
amd64 by checking the pointer size. Instead we can get the target
architecture for the compilier and check that to distinguish between
architectures.

llvm/lib/DebugInfo/PDB/CMakeLists.txt

@@ -6,7 +6,12 @@ endmacro()
 if(LLVM_ENABLE_DIA_SDK)
   include_directories(${MSVC_DIA_SDK_DIR}/include)
   set(LIBPDB_LINK_FOLDERS "${MSVC_DIA_SDK_DIR}\\lib")
-  if (CMAKE_SIZEOF_VOID_P EQUAL 8)
+
+  if ("$ENV{VSCMD_ARG_TGT_ARCH}" STREQUAL "arm64")
+    set(LIBPDB_LINK_FOLDERS "${LIBPDB_LINK_FOLDERS}\\arm64")
+  elseif ("$ENV{VSCMD_ARG_TGT_ARCH}" STREQUAL "arm")
+    set(LIBPDB_LINK_FOLDERS "${LIBPDB_LINK_FOLDERS}\\arm")
+  elseif (CMAKE_SIZEOF_VOID_P EQUAL 8)
     set(LIBPDB_LINK_FOLDERS "${LIBPDB_LINK_FOLDERS}\\amd64")
   endif()
   file(TO_CMAKE_PATH "${LIBPDB_LINK_FOLDERS}\\diaguids.lib" LIBPDB_ADDITIONAL_LIBRARIES)