Added --platform option to build command

[nobu]

Description:

@headius wrote at ruby/io-console#11 (comment):

One bit of process I'm not sure about: this -java gem can only be built with JRuby, and likewise the standard gem can't be build on JRuby, because the gemspec uses RUBY_ENGINE. This could be modified to allow building both versions on any impl, since they're just packaging files (i.e. there's no compile step required for either JRuby or MRI).

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