Fix breakage where builds except `host` don't refer` mrbc` in `host`

Build error occurred under the following conditions.

* `host` build is presym enabled and unspecified `mruby-bin-mrbc` gem.
* Build except `host` unspecified `mruby-bin-mrbc` gem and mrbcfile.

### Example

#### Build configuration

```ruby
MRuby::Build.new{|conf| conf.toolchain}
MRuby::Build.new("host2"){|conf| conf.toolchain}
```

#### Error

```console
rake aborted!
external mrbc or mruby-bin-mrbc gem in current('host2') or 'host' build is required
/mruby/lib/mruby/build.rb:326:in `mrbcfile'
/mruby/tasks/mrblib.rake:9:in `block in <top (required)>'
/mruby/lib/mruby/build.rb:18:in `instance_eval'
/mruby/lib/mruby/build.rb:18:in `block in each_target'
/mruby/lib/mruby/build.rb:17:in `each'
/mruby/lib/mruby/build.rb:17:in `each_target'
/mruby/tasks/mrblib.rake:1:in `<top (required)>'
/mruby/Rakefile:27:in `load'
/mruby/Rakefile:27:in `<top (required)>'
```

0 files changed, 0 insertions, 0 deletions