[D-runtime] druntime fixes again break Windows DLL support

[Walter Bright]

..\dmd mydll.obj ..\druntime\lib\gcstub.obj mydll.def -L/map
OPTLINK (R) for Win32  Release 8.00.12
Copyright (C) Digital Mars 1989-2010  All rights reserved.
http://www.digitalmars.com/ctg/optlink.html
mydll.obj(mydll)
  Error 42: Symbol Undefined 
_D4core10dll_helper18dll_process_attachFT4core3sys7windows7windows6HANDL
EbZb
mydll.obj(mydll)
  Error 42: Symbol Undefined 
_D4core10dll_helper18dll_process_detachFT4core3sys7windows7windows6HANDL
EbZv
mydll.obj(mydll)
  Error 42: Symbol Undefined _D4core10dll_helper12__ModuleInfoZ
--- errorlevel 3