tcc.c: fix previous commit "Use CString to concat linker options"

- remove redunant else branch
- zero-terminate linker_arg
- declare cstr_xxx as PUB_FUNC
  (which are functions used in tcc.c but not in the libtcc API.
   Useful for a tcc(.exe) that uses the libtcc.(so/dll))
- while at it, export PUB_FUNCs from dll

win32/build-tcc.bat

@@ -34,7 +34,7 @@ copy ..\libtcc.h libtcc\libtcc.h
 tiny_impdef libtcc.dll -o lib/libtcc.def
 
 :tcc
-%CC% %target% -DONE_SOURCE ../tcc.c -o tcc.exe -ltcc -Llibtcc
+%CC% %target% ../tcc.c -o tcc.exe -ltcc -Llibtcc
 
 :copy_std_includes
 copy ..\include\*.h include