Ractor: lock around global variable get/set

There's a global id_table `rb_global_tbl` that needs a lock (I used VM lock). In the future, we might use a lock-free rb_id_table if we create such a data structure.

Reproduction script that might crash or behave strangely:

```ruby
100.times do
  Ractor.new do
    1_000_000.times do
      $stderr
      $stdout
      $stdin
      $VERBOSE
      $stderr
      $stdout
      $stdin
      $VERBOSE
      $stderr
      $stdout
      $stdin
      $VERBOSE
    end
  end
end

$myglobal0 = nil;
$myglobal1 = nil;
  # ... vim macros to the rescue
$myglobal100000 = nil;
```

bootstraptest/test_ractor.rb

@@ -588,8 +588,8 @@ assert_equal 'true', %q{
   r.value[:frozen]
 }
 
-# Access to global-variables are prohibited
+# Access to global-variables are prohibited (read)
-assert_equal 'can not access global variables $gv from non-main Ractors', %q{
+assert_equal 'can not access global variable $gv from non-main Ractor', %q{
   $gv = 1
   r = Ractor.new do
     $gv
@@ -602,8 +602,8 @@ assert_equal 'can not access global variables $gv from non-main Ractors', %q{
   end
 }
 
-# Access to global-variables are prohibited
+# Access to global-variables are prohibited (write)
-assert_equal 'can not access global variables $gv from non-main Ractors', %q{
+assert_equal 'can not access global variable $gv from non-main Ractor', %q{
   r = Ractor.new do
     $gv = 1
   end

variable.c

@@ -584,6 +584,7 @@ rb_find_global_entry(ID id)
     struct rb_global_entry *entry;
     VALUE data;
 
+    RB_VM_LOCKING() {
         if (!rb_id_table_lookup(rb_global_tbl, id, &data)) {
             entry = NULL;
         }
@@ -591,9 +592,10 @@ rb_find_global_entry(ID id)
             entry = (struct rb_global_entry *)data;
             RUBY_ASSERT(entry != NULL);
         }
+    }
 
     if (UNLIKELY(!rb_ractor_main_p()) && (!entry || !entry->ractor_local)) {
-        rb_raise(rb_eRactorIsolationError, "can not access global variables %s from non-main Ractors", rb_id2name(id));
+        rb_raise(rb_eRactorIsolationError, "can not access global variable %s from non-main Ractor", rb_id2name(id));
     }
 
     return entry;
@@ -621,7 +623,9 @@ rb_gvar_undef_compactor(void *var)
 static struct rb_global_entry*
 rb_global_entry(ID id)
 {
-    struct rb_global_entry *entry = rb_find_global_entry(id);
+    struct rb_global_entry *entry;
+    RB_VM_LOCKING() {
+        entry = rb_find_global_entry(id);
         if (!entry) {
             struct rb_global_variable *var;
             entry = ALLOC(struct rb_global_entry);
@@ -641,6 +645,7 @@ rb_global_entry(ID id)
             var->namespace_ready = false;
             rb_id_table_insert(rb_global_tbl, id, (VALUE)entry);
         }
+    }
     return entry;
 }
 
@@ -1003,6 +1008,7 @@ rb_gvar_set(ID id, VALUE val)
     struct rb_global_entry *entry;
     const rb_namespace_t *ns = rb_current_namespace();
 
+    RB_VM_LOCKING() {
         entry = rb_global_entry(id);
 
         if (USE_NAMESPACE_GVAR_TBL(ns, entry)) {
@@ -1013,6 +1019,7 @@ rb_gvar_set(ID id, VALUE val)
         else {
             retval = rb_gvar_set_entry(entry, val);
         }
+    }
     return retval;
 }
 
@@ -1026,9 +1033,11 @@ VALUE
 rb_gvar_get(ID id)
 {
     VALUE retval, gvars, key;
+    const rb_namespace_t *ns = rb_current_namespace();
+    // TODO: use lock-free rb_id_table when it's available for use (doesn't yet exist)
+    RB_VM_LOCKING() {
         struct rb_global_entry *entry = rb_global_entry(id);
         struct rb_global_variable *var = entry->var;
-    const rb_namespace_t *ns = rb_current_namespace();
 
         if (USE_NAMESPACE_GVAR_TBL(ns, entry)) {
             gvars = ns->gvar_tbl;
@@ -1047,6 +1056,7 @@ rb_gvar_get(ID id)
         else {
             retval = (*var->getter)(entry->id, var->data);
         }
+    }
     return retval;
 }
 
@@ -1128,7 +1138,7 @@ rb_f_global_variables(void)
 void
 rb_alias_variable(ID name1, ID name2)
 {
-    struct rb_global_entry *entry1, *entry2;
+    struct rb_global_entry *entry1 = NULL, *entry2;
     VALUE data1;
     struct rb_id_table *gtbl = rb_global_tbl;
 
@@ -1136,6 +1146,7 @@ rb_alias_variable(ID name1, ID name2)
         rb_raise(rb_eRactorIsolationError, "can not access global variables from non-main Ractors");
     }
 
+    RB_VM_LOCKING() {
         entry2 = rb_global_entry(name2);
         if (!rb_id_table_lookup(gtbl, name1, &data1)) {
             entry1 = ALLOC(struct rb_global_entry);
@@ -1152,12 +1163,12 @@ rb_alias_variable(ID name1, ID name2)
                 free_global_variable(var);
             }
         }
-    else {
+        if (entry1) {
-        return;
-    }
             entry2->var->counter++;
             entry1->var = entry2->var;
         }
+    }
+}
 
 static void
 IVAR_ACCESSOR_SHOULD_BE_MAIN_RACTOR(ID id)