Allow `break` in `build_config`.

If I break out of a block given to `MRuby::Build.new` with `break` or `throw`, I will get a seemingly inexplicable error because the `presym`-related initialization is not done.

```console
% cat build_config1.rb
MRuby::Build.new do
  toolchain
  break
end

% rake CONFIG=build_config1.rb
rake aborted!
external mrbc or mruby-bin-mrbc gem in current('host') or 'host' build is required
/var/tmp/mruby/lib/mruby/build.rb:332:in `mrbcfile'
/var/tmp/mruby/tasks/mrblib.rake:9:in `block in <top (required)>'
/var/tmp/mruby/lib/mruby/build.rb:18:in `instance_eval'
/var/tmp/mruby/lib/mruby/build.rb:18:in `block in each_target'
/var/tmp/mruby/lib/mruby/build.rb:17:in `each'
/var/tmp/mruby/lib/mruby/build.rb:17:in `each_target'
/var/tmp/mruby/tasks/mrblib.rake:1:in `<top (required)>'
/var/tmp/mruby/Rakefile:27:in `load'
/var/tmp/mruby/Rakefile:27:in `<top (required)>'
(See full trace by running task with --trace)
```

If a non-exceptional global jump occurs, it can be initialized by `ensure` to solve this problem.

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