[C++] Avoid one place of frontend calling into targetm

[Eric Botcazou]

> The patch is trying to solve the problem by moving code 4) into
> callgraph so all frontends can use the infrastructure and moving the
> test for static_ctors_dtors out of C++ frontend (cgraph should abstract
> this completely now).

I'm in the process of converting the Ada front-end to the new scheme, but I 
note that this will be a small step backwards because in ada/utils.c:

/* Arrays of functions called automatically at the beginning and
   end of execution, on targets without .ctors/.dtors sections.  */
static GTY(()) VEC(tree,gc) *static_ctors;
static GTY(()) VEC(tree,gc) *static_dtors;

and now in cgraphunit.c:

static GTY (()) tree static_ctors;
static GTY (()) tree static_dtors;

-- 
Eric Botcazou