[lldb] fix RPATH when linking against Python3.framework

The install name for the Python 3 framework in Xcode is relative to
the framework's location and not the dylib itself.

  @rpath/Python3.framework/Versions/3.x/Python3

This means that we need to compute the path to the Python3.framework
and use that as the RPATH instead of the usual dylib's directory.