Create art_debug_opt_flag soong config variable

For use in art_debug_defaults.

Bug: 247785938
Test: See other cl in topic
Change-Id: I3d272b893ba05d65f21fdf65bf4f06dc172adfd8
diff --git a/core/android_soong_config_vars.mk b/core/android_soong_config_vars.mk
index 718adb5..c52fa92 100644
--- a/core/android_soong_config_vars.mk
+++ b/core/android_soong_config_vars.mk
@@ -101,6 +101,9 @@
 endif
 
 $(call soong_config_set,art_module,source_build,$(ART_MODULE_BUILD_FROM_SOURCE))
+ifdef ART_DEBUG_OPT_FLAG
+$(call soong_config_set,art_module,art_debug_opt_flag,$(ART_DEBUG_OPT_FLAG))
+endif
 
 ifdef TARGET_BOARD_AUTO
   $(call add_soong_config_var_value, ANDROID, target_board_auto, $(TARGET_BOARD_AUTO))