Add Cross GCC 4.8.1 for Linux 64bit cask
user
```ruby
class Gcc481Linux64 < Cask
  url 'http://crossgcc.rts-software.org/download/gcc-4.8.1-for-linux32-linux64/gcc-4.8.1-for-linux64.dmg'
  homepage 'http://crossgcc.rts-software.org/doku.php?id=start'
  version '4.8.1'
  sha1 'b25102b4f8bb4cebb042c64bb6ea823ce4d22058'
  install 'gcc-4.8.1-for-linux64.pkg'
end
```
assistant