build: make -O3 actually reach the optimized runtime

CMake appends CMAKE_CXX_FLAGS_<CONFIG> after CMAKE_CXX_FLAGS, and clang
honours the last -O on the command line. AGP builds the release variant as
RelWithDebInfo, whose default flags are "-O2 -g -DNDEBUG", so the -O3 set in
CMAKE_CXX_FLAGS was overridden and every shipped runtime was compiled at -O2.

Append -O3 to CMAKE_CXX_FLAGS_RELWITHDEBINFO inside the same branch so it
comes last. The -O3 in CMAKE_CXX_FLAGS stays: CMAKE_CXX_FLAGS_DEBUG carries
no -O, so a Debug-config build with -Poptimized still needs it.

Author: edusperoni

test-app/runtime/CMakeLists.txt

@@ -63,6 +63,11 @@ include_directories(
 # must carry the release flags. Only a local Debug build keeps plain -g.
 if (OPTIMIZED_BUILD OR OPTIMIZED_WITH_INSPECTOR_BUILD OR NOT CMAKE_BUILD_TYPE STREQUAL "Debug")
     set(CMAKE_CXX_FLAGS "${COMMON_CMAKE_ARGUMENTS} -O3 -fvisibility=hidden -ffunction-sections -fno-data-sections")
+    # CMake appends CMAKE_CXX_FLAGS_<CONFIG> after CMAKE_CXX_FLAGS and clang honours the
+    # last -O, so the -O3 above only survives in configs that carry no -O of their own
+    # (Debug). AGP builds the release variant as RelWithDebInfo, whose default -O2 would
+    # otherwise win.
+    string(APPEND CMAKE_CXX_FLAGS_RELWITHDEBINFO " -O3")
 else ()
     set(CMAKE_CXX_FLAGS "${COMMON_CMAKE_ARGUMENTS} -g")
 endif ()