Add debug version of RTL and allow easy switching

Originally raised here: freepascal.org/lazarus/lazarus#39574

What happens

The shipped RTL is a release version. It does not have debug info, does not support stepping in, stack trace line numbers, etc. When e.g. TStringList.LoadFromFile raises an exception, debugger does not shows where it's raised.

What did you expect

Standard 'debug' compilation should be against the debug version of RTL. Project options should include the switch for that. Attached screenshot of this function in Delphi.

Steps to reproduce

TStringList.Create().LoadFromFile('qwe');

https://gitlab.com/freepascal.org/lazarus/lazarus/uploads/f84ba1ed2f572da4c45599855222ce3d/image.png