Add support for configurable -OX optimization levels for debug builds

runtime/BUILD.gn

Index: runtime/BUILD.gn
diff --git a/runtime/BUILD.gn b/runtime/BUILD.gn
index 36983b6a75caede18bcd48379045163686f254f8..05d33f76211d3b16a2702c7c6dcd98f359bc6363 100644
--- a/runtime/BUILD.gn
+++ b/runtime/BUILD.gn
@@ -28,6 +28,9 @@ declare_args() {
   dart_target_arch = ""
 
   dart_experimental_interpreter = false
+
+  # The optimization level to use for debug builds.
+  dart_debug_optimization_level = "2"
 }
 
 config("dart_public_config") {
@@ -163,7 +166,7 @@ config("dart_config") {
     ]
 
     if (dart_debug) {
-      cflags += [ "-O2" ]
+      cflags += [ "-O${dart_debug_optimization_level}" ]
     } else {
       cflags += [ "-O3" ]
     }