compiler: audit debug mode checks

* Introduce `-Ddebug-extensions` for enabling compiler debug helpers
 * Replace safety mode checks with `std.debug.runtime_safety`
 * Replace debugger helper checks with `!builtin.strip_debug_info`

Sometimes, you just have to debug optimized compilers...

lib/std/multi_array_list.zig

@@ -574,7 +574,7 @@ pub fn MultiArrayList(comptime T: type) type {
         }
 
         comptime {
-            if (builtin.mode == .Debug) {
+            if (!builtin.strip_debug_info) {
                 _ = &dbHelper;
                 _ = &Slice.dbHelper;
             }