Fix various autoload race conditions. (ruby#5898)

* Add RUBY_VM_CRITICAL_SECTION for detecting unexpected context switch.

* Prevent race between GC mark and autoload setup.

* Protect race on autoload state.

* Avoid potential race condition when allocating `autoload_featuremap`.

* Add NEWS entry for autoload fixes.

NEWS.md

@@ -208,6 +208,8 @@ The following deprecated APIs are removed.
 
 ## Implementation improvements
 
+* Fixed several race conditions in `Kernel#autoload`. [[Bug #18782]]
+
 ## JIT
 
 ### MJIT
@@ -253,3 +255,4 @@ The following deprecated APIs are removed.
 [Feature #18598]: https://bugs.ruby-lang.org/issues/18598
 [Bug #18625]:     https://bugs.ruby-lang.org/issues/18625
 [Bug #18633]:     https://bugs.ruby-lang.org/issues/18633
+[Bug #18782]:     https://bugs.ruby-lang.org/issues/18782

thread.c

@@ -1562,6 +1562,10 @@ static inline int
 blocking_region_begin(rb_thread_t *th, struct rb_blocking_region_buffer *region,
 		      rb_unblock_function_t *ubf, void *arg, int fail_if_interrupted)
 {
+#ifdef RUBY_VM_CRITICAL_SECTION
+    VM_ASSERT(rb_vm_critical_section_entered == 0);
+#endif
+
     region->prev_status = th->status;
     if (unblock_function_set(th, ubf, arg, fail_if_interrupted)) {
 	th->blocking_region_buffer = region;