… platform.

Also prefix the selection of rbconfig in config.yml with the corresponding
Ruby platform.

This allowes to use cross compiled Ruby-builds for multiple target
architectures on the same machine. This is especially usefull to compile
for Ruby-2.0 i386-mingw32 and x64-mingw32 platforms.

This is especially useful to set distinct library paths for different
target platforms.

Building gems for distinct platforms within one rake run conflicted
in the lib/ directory, because all binary extensions were stored
there and were copied from there to become packaged.
This resulted in storing a binary extension of a wrong platform in
a gem.

To avoid this, the files to package are now stored in a platform specific
stage directory and are used to package the specific gem.