[build] -fno-strict-vtable-pointers is a Clang-only flag.

TEST=ci
Bug: https://github.com/dart-lang/sdk/issues/48703
Change-Id: I76dc6abe2ad709b5ae6c9df31db5174774b8ae9f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/239324
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/BUILD.gn

@@ -203,11 +203,13 @@ config("dart_config") {
       "-ggdb3",
       "-fno-rtti",
       "-fno-exceptions",
-      "-fno-strict-vtable-pointers",  # Handle assignment updates vtable
-                                      # pointers.
     ]
     if (is_clang) {
-      cflags += [ "-Wimplicit-fallthrough" ]
+      cflags += [
+        "-Wimplicit-fallthrough",
+        "-fno-strict-vtable-pointers",  # Handle assignment updates vtable
+                                        # pointers.
+      ]
     } else {
       cflags += [ "-Wno-cast-function-type" ]
     }