Fix race condition in method invalidation for Ractors

We lock the VM to invalidate method entries. However, we do not lock the
VM to call methods, so it's possible that during a method call the method
entry gets invalidated. We only check that the method entry in the callcache
is not invalidated at the beginning of the method call, which makes it
possible to have race conditions. This causes crashes like:

    vm_callinfo.h:421: Assertion Failed: vm_cc_cme:cc->klass != Qundef || !vm_cc_markable(cc)
    vm_insnhelper.c:2200: Assertion Failed: vm_lookup_cc:!METHOD_ENTRY_INVALIDATED(vm_cc_cme(ccs_cc))

This commit adds a VM barrier to method cache invalidation to ensure that
other Ractors are stopped at a safe-point before invalidating the method
entry.

bootstraptest/test_ractor.rb

@@ -1573,6 +1573,33 @@ assert_equal 'true', %q{
   rs.map{|r| r.value} == Array.new(RN){n}
 }
 
+# check method cache invalidation
+assert_equal 'true', %q{
+  class Foo
+    def hello = nil
+  end
+
+  r1 = Ractor.new do
+    1000.times do
+      class Foo
+        def hello = nil
+      end
+    end
+  end
+
+  r2 = Ractor.new do
+    1000.times do
+      o = Foo.new
+      o.hello
+    end
+  end
+
+  r1.value
+  r2.value
+
+  true
+}
+
 # check experimental warning
 assert_match /\Atest_ractor\.rb:1:\s+warning:\s+Ractor is experimental/, %q{
   Warning[:experimental] = $VERBOSE = true

vm_callinfo.h

@@ -613,7 +613,7 @@ static inline bool
 vm_cc_check_cme(const struct rb_callcache *cc, const rb_callable_method_entry_t *cme)
 {
     bool valid;
-    RB_VM_LOCKING() {
+    RB_VM_LOCKING_NO_BARRIER() {
         valid = vm_cc_cme(cc) == cme ||
             (cme->def->iseq_overload && vm_cc_cme(cc) == rb_vm_lookup_overloaded_cme(cme));
     }

vm_method.c

@@ -428,6 +428,8 @@ clear_method_cache_by_id_in_class(VALUE klass, ID mid)
     if (rb_objspace_garbage_object_p(klass)) return;
 
     RB_VM_LOCKING() {
+        rb_vm_barrier();
+
         if (LIKELY(RCLASS_SUBCLASSES_FIRST(klass) == NULL)) {
             // no subclasses
             // check only current class
@@ -1752,6 +1754,8 @@ cached_callable_method_entry(VALUE klass, ID mid)
             return ccs->cme;
         }
         else {
+            rb_vm_barrier();
+
             rb_managed_id_table_delete(cc_tbl, mid);
             rb_vm_ccs_invalidate_and_free(ccs);
         }