Fix I_HOLD_LOCK assertion violation in child process

Issue #682 (bdwgc).

Update `GC_lock_holder` in `fork_child_proc()` as value of
`thread_id_self()` might differ from that of the parent process.

* include/private/gc_locks.h [(NN_PLATFORM_CTR || NINTENDO_SWITCH)
&& GC_ASSERTIONS] (SET_LOCK_HOLDER): Define macro (to no-op).
* pthread_support.c [CAN_HANDLE_FORK] (fork_child_proc): Call
`SET_LOCK_HOLDER()` at the beginning of function.

include/private/gc_locks.h

@@ -36,6 +36,9 @@ extern void GC_lock(void);
 extern void GC_unlock(void);
 #    define UNCOND_LOCK() GC_lock()
 #    define UNCOND_UNLOCK() GC_unlock()
+#    ifdef GC_ASSERTIONS
+#      define SET_LOCK_HOLDER() (void)0
+#    endif
 #  endif
 
 #  if (!defined(AO_HAVE_test_and_set_acquire) || defined(GC_WIN32_THREADS) \

pthread_support.c

@@ -1519,6 +1519,11 @@ GC_ATTR_NO_SANITIZE_THREAD
 static void
 fork_child_proc(void)
 {
+#    ifdef GC_ASSERTIONS
+  /* Update GC_lock_holder as value of thread_id_self() might       */
+  /* differ from that of the parent process.                        */
+  SET_LOCK_HOLDER();
+#    endif
   GC_release_dirty_lock();
 #    ifndef GC_DISABLE_INCREMENTAL
   GC_dirty_update_child();