Build: Fix c++ build for Armclang

Change-Id: Ib48acd86d5eb44b1d2a257ce10e099858e5ddf3f
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/toolchain_ARMCLANG.cmake b/toolchain_ARMCLANG.cmake
index e6a6a95..afd24c6 100644
--- a/toolchain_ARMCLANG.cmake
+++ b/toolchain_ARMCLANG.cmake
@@ -202,6 +202,7 @@
 
     if (DEFINED TFM_SYSTEM_PROCESSOR)
         set(CMAKE_C_FLAGS "-mcpu=${CMAKE_SYSTEM_PROCESSOR}")
+        set(CMAKE_CXX_FLAGS "-mcpu=${CMAKE_SYSTEM_PROCESSOR}")
         set(CMAKE_C_LINK_FLAGS   "--cpu=${CMAKE_SYSTEM_PROCESSOR}")
         set(CMAKE_CXX_LINK_FLAGS "--cpu=${CMAKE_SYSTEM_PROCESSOR}")
         set(CMAKE_ASM_LINK_FLAGS "--cpu=${CMAKE_SYSTEM_PROCESSOR}")