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;
```

0 files changed, 0 insertions, 0 deletions