Default CMAKE_SYSTEM_PROCESSOR to the host (#10912)

### Summary

Although by default cmake use the host's architecture if
`CMAKE_SYSTEM_PROCESSOR` isn't provided, it doesn't actually set the
value. This can be an issue in cases where people try to read this value
(i.e.
[extension/llm/custom_ops/CMakeLists.txt](https://github.com/pytorch/executorch/blob/0a6f6220dd7252f5b79bf7b33edf1ea0631158f3/extension/llm/custom_ops/CMakeLists.txt#L52-L66)).
So, let's just explicitly set it.

### Test plan

CI

cc @larryliu0820

Author: jathu

CMakeLists.txt

@@ -54,6 +54,11 @@ if(NOT CMAKE_CXX_STANDARD)
 endif()
 announce_configured_options(CMAKE_CXX_STANDARD)
 
+if(NOT CMAKE_SYSTEM_PROCESSOR)
+  set(CMAKE_SYSTEM_PROCESSOR ${CMAKE_HOST_SYSTEM_PROCESSOR})
+endif()
+announce_configured_options(CMAKE_SYSTEM_PROCESSOR)
+
 if(NOT CMAKE_BUILD_TYPE)
   set(CMAKE_BUILD_TYPE Debug)
 endif()