The Ada runtime can now be set with the ADA_RTS variable. The ada
library builds a (currently) minimal runtime from the gcc sources that
come with Genode (to stay consistent with the used compiler) and sets
the runtime path accordingly. It is build as a shared library ada.lib.so
which needs to be added to the build files.
I split the existing Ada test into program and library, and moved it
to libports as it depends on the runtime library residing in this
repository too.
Fixes#2748
