* Changed linking infrastructure to look like assembler one:
  * 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: http://svn.freepascal.org/svn/fpc/trunk@25279 3ad0048d-3df7-0310-abae-a5850022a9f2