Deprioritize top level -O2 in CMAKE_CXX_FLAGS_RELEASE (#9394)

By prepending rather than appending to CMAKE_CXX_FLAGS_RELEASE, it
allows to specify another optimization level earlier in the build
process and still have that take precedence over the -O2.

Author: AdrianLundell

CMakeLists.txt

@@ -1,6 +1,6 @@
 # Copyright (c) Meta Platforms, Inc. and affiliates.
-# Copyright 2024-2025 Arm Limited and/or its affiliates.
 # All rights reserved.
+# Copyright 2024-2025 Arm Limited and/or its affiliates.
 #
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
@@ -161,7 +161,7 @@ if(OPTIMIZE_SIZE)
   set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -Os")
 else()
   # -O2: Moderate opt.
-  set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -O2")
+  set(CMAKE_CXX_FLAGS_RELEASE "-O2 ${CMAKE_CXX_FLAGS_RELEASE}")
 endif()
 
 option(EXECUTORCH_BUILD_ANDROID_JNI "Build Android JNI" OFF)