[fuchsia, arm64] Use mx_cache_flush instead of removed __clear_cache.

runtime/vm/flag_list.h

Index: runtime/vm/flag_list.h
diff --git a/runtime/vm/flag_list.h b/runtime/vm/flag_list.h
index 984d2275f74629116e583017f6551dc0033cf9f2..f2fe03ba28b7d3ac9ae4cbb20083706e61391859 100644
--- a/runtime/vm/flag_list.h
+++ b/runtime/vm/flag_list.h
@@ -26,6 +26,13 @@
 #define USING_PRODUCT false
 #endif
 
+// Don't use USING_FUCHSIA outside of this file.
+#if defined(HOST_OS_FUCHSIA)
+#define USING_FUCHSIA true
+#else
+#define USING_FUCHSIA false
+#endif
+
 // List of all flags in the VM.
 // Flags can be one of three categories:
 // * P roduct flags: Can be set in any of the deployment modes, including in
@@ -48,7 +55,7 @@
     "Stress test system: stop background compiler often.")                     \
   R(break_at_isolate_spawn, false, bool, false,                                \
     "Insert a one-time breakpoint at the entrypoint for all spawned isolates") \
-  C(collect_code, false, true, bool, true,                                     \
+  C(collect_code, false, !USING_FUCHSIA, bool, !USING_FUCHSIA,                 \
     "Attempt to GC infrequently used code.")                                   \
   P(collect_dynamic_function_names, bool, true,                                \
     "Collects all dynamic function names to identify unique targets")          \