Support x64 builds

The following changes enhance the build process of Ruby and cross-compilation, ensuring the cross-compiled gems will work with newer versions of RubyInstaller.

I'm opening this pull request to discuss the modifications. I'm not a bash guru and the script might be brittle, but seems to be getting the job done.

I've implemented a separate `prepare_xrubies.sh` script to compile all the versions as it depends on different compilers.

Right now, it compiles the following:

- 1.8.7 with `i586-mingw32msvc` compiler (bundled with Ubuntu)
- 1.9.3 with `i686-w64-mingw32` compiler (GCC 4.7.2 from mingw-w64 project)
- 2.0.0 with `i686-w64-mingw32` compiler (same as above)
- 2.0.0 with `x86_64-w64-mingw32` compiler (64bits of the same compiler)

It still requires:

- [x] Add 2.0.0 to `RUBY_CC_VERSION` for the other scripts
- [x] Avoid mingw-w64 be downloaded multiple times
- [x] Avoid `$PATH` being altered every time