Cast StacktraceSignalHandler to extern "C" to avoid the warning:

 SystemInformation.cxx, line 3048: Warning (Anachronism): Assigning
  void(*)(int,siginfo*,void*) to extern "C" void(*)(int,siginfo*,void*)

on the Sun compiler.

Change-Id: I8f8ac99656bc8ee6f604ea69c415a093699ca86d