* Changed direct references to linker classes in TSystemInfo records to enumerated ids.
* RegisterExtennalLinker and RegisterInternalLinker procedures replaced by single RegisterLinker procedure and moved to link.pas.
- TAbstractLinker is no longer necessary, removed.
This change allows to register linker once per id, rather than once per target, and also allows TSystemInfo records to be read-only.
git-svn-id: trunk@25279 -