cmake: Fallback if the target is not supported

It is user friendlier.

Change-Id: I3eb1ca69464d75da4d81e4139e0384d97e0ccac1

build/cmake/aom_configure.cmake

diff --git a/build/cmake/aom_configure.cmake b/build/cmake/aom_configure.cmake
index af19a7e..c62ac70 100644
--- a/build/cmake/aom_configure.cmake
+++ b/build/cmake/aom_configure.cmake
@@ -71,8 +71,9 @@
   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}")
+    message(WARNING "The architecture ${CMAKE_SYSTEM_PROCESSOR} is not "
+            "supported, falling back to the generic target")
+    set(AOM_TARGET_CPU "generic")
   endif ()
 endif ()