+ added tai_const.Create_nil_codeptr and .Create_nil_dataptr, which should eventually replace tai_const.Create_sym(nil)

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@24976 3ad0048d-3df7-0310-abae-a5850022a9f2
* use create_nil_dataptr instead of create_sym(nil) in TRTTIWriter.write_rtti_reference; among other things, this fixes writeln(enum) in the i8086 medium memory model

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@24977 3ad0048d-3df7-0310-abae-a5850022a9f2