Mark asan fake stacks during machine stack marking

ASAN leaves a pointer to the fake frame on the stack; we can use the
__asan_addr_is_in_fake_stack API to work out the extent of the fake
stack and thus mark any VALUEs contained therein.

[Bug #20001]

thread.c

@@ -525,6 +525,9 @@ void
 ruby_thread_init_stack(rb_thread_t *th, void *local_in_parent_frame)
 {
     native_thread_init_stack(th, local_in_parent_frame);
+#ifdef RUBY_ASAN_ENABLED
+    th->asan_fake_stack_handle = asan_get_thread_fake_stack_handle();
+#endif
 }
 
 const VALUE *