Change extensions dir order.

There are several reasons to do that:

- It will preserve the current state, i.e. there is folder with
  full_name and under that folder, there is the extension, without any
  addition clutter.

- If we consider other alternative Ruby implementations such as Rubinius,
  it speaks in favor of this proposal as well. It means I could go into
  Rubinius folder and rebuild every gem extension I'll found. The opposite
  way, such as go into each gem folder is less favourable (although
  possible). Additionally, if I remove Rubinius, I can simple delete whole
  directory branch instead of iterating through the folders.

- It will ease packaging. I.e. the RubyGems package owns the platform
  specific folders, the gem package owns just its own sub-directory with
  its content. In current state, the platform specific of the gem would
  need to share ownership of the gem directory, which is repetitive.