Ask rubygems if the gemspec is supported on the current platform

RUBY_PLATFORM was i368-msys32, gemspec was x86-msys32
A few things:

* Can we get rid of the negation on the `if` and just switch the order in which the cases are written?
* We're doing quite a lot in this method now, we should break it up:

```ruby
def executables
  if have_gemspec?
    executables_from_gemspec
  else
    executables_from_glob
  end
end

def executables_from_gemspec
  most_specific_gemspec.executables.map # etc
end

def most_specific_gemspec
  # etc.
end
```