Provide possibility to generate an indirect VMT symbol name.

symdef.pas, tobjectdef:
  * vmt_mangledname: add new parameter "indirect" (default "false") which triggers the generation of an indirect symbol name

git-svn-id: http://svn.freepascal.org/svn/fpc/branches/svenbarth@28339 3ad0048d-3df7-0310-abae-a5850022a9f2
Generate an indirect VMT symbol for each generated VMT.

ncgvmt.pas, TVMTWriter:
  * writevmt: write a symbol with the indirect VMT name that references the direct VMT symbol

git-svn-id: http://svn.freepascal.org/svn/fpc/branches/svenbarth@28340 3ad0048d-3df7-0310-abae-a5850022a9f2