Fix Kernel#global_variables for $1-$9

This patch fix wrong symbols for global variables $1-$9.

Before:
```ruby
> global_variables
 => [:"\000\000", :"\000\000", :"\000\000", :"\000\000", :"\000\000", :"\000\000", :"\000\000", :"\000\000", :"\000\000"]
```

After:
```ruby
> global_variables
 => [:$1, :$2, :$3, :$4, :$5, :$6, :$7, :$8, :$9]
```

ps. I'm new with mruby, may be this variables should be assigned in a different place. This global variables assign when we call `global_variables`

```ruby
list1 = Symbol.all_symbols.dup
global_variables
p Symbol.all_symbols - list1
```

Without patch: 
```
[:"\177\000", :"\177\000", :"\177\000", :"\177\000", :"\177\000", :"\177\000"]
```

With patch:
```
[:$6, :$3, :$1, :$2, :$5, :$7, :$8, :$9, :$4]
```