cmake: Always set the architecture

This way the RTDC check would trigger instead of passing silently
and produce a broken header file with a confusing message.

Change-Id: Ic8f9885c7d879186bdfbf8e99f8d4c1e6079dc70
diff --git a/build/cmake/aom_configure.cmake b/build/cmake/aom_configure.cmake
index fddd1ba..af19a7e 100644
--- a/build/cmake/aom_configure.cmake
+++ b/build/cmake/aom_configure.cmake
@@ -70,6 +70,9 @@
     set(AOM_TARGET_CPU "${CMAKE_SYSTEM_PROCESSOR}")
   elseif ("${CMAKE_SYSTEM_PROCESSOR}" MATCHES "aarch64")
     set(AOM_TARGET_CPU "arm64")
+  else ()
+    message(WARNING "The architecture ${CMAKE_SYSTEM_PROCESSOR} is not supported")
+    set(AOM_TARGET_CPU "${CMAKE_SYSTEM_PROCESSOR}")
   endif ()
 endif ()