As in summary.  Because thunks do not set up the PIC register, going through the
PLT causes a SEGV in best case.  Another PIC construct than for normal calls
must be used, but unfortunately the only (currently) available PC-relative PIC
reloc for cris-axis-linux-gnu requests a PLT.  The thunked function is global. 
Using a local symbol makes the linker omit the redirection through the PLT and
the jump goes directly to the right place.

On trunk and 3.4 branch, the thunk uses a local symbol (equated to the thunked
function, generated by generic code), so TRT happens.
See <URL:http://gcc.gnu.org/ml/gcc-patches/2003-06/msg02603.html> for
the first of the patch where the generic local-thunk was implemented (needed
follow-up patches for correctness).  I'll go with a target-local patch.

Testcase:
-------------- th1.h
class B
{
  int bb1;
  int bb2;
 public:
  virtual void b();
};
extern class B* mka();
-------------- th1.cc (compile and link with -fpic -shared -o libth1.so)
#include "th1.h"
class A
{
  int aa;
public:
  virtual void a() = 0;
};

class C : public A, public B
{
  int cc;
public:
  virtual void b();
  virtual void a();
};

extern "C" void abort (void);
extern "C" void exit (int);

class B* mka (void)
{
  return new C;
}

void C::b(void)
{
  exit (0);
}

void B::b(void)
{
  abort ();
}

void C::a(void)
{
  abort ();
}
-------------- m.cc (compile and link with -L. -lth1, expect normal exit)

#include "th1.h"
extern "C" void abort (void);
int ii[10];
int main(void)
{
  register int picreg
#if defined (__CRIS__) && !defined (__PIC__) && !defined (__pic__)
    __asm__ ("r0")
#endif
    = -1;
  class B *bb = mka();
  bb->b();
  // Make sure picreg is alive and used after bb->b() above.
  asm ("" : : "r" (picreg));
  abort ();
}