AP_HAL: generate a core file for a given PID

Similarly to dumpstack.sh, uses gdb to dump a core file

libraries/AP_HAL/system.cpp

@@ -9,3 +9,7 @@ void WEAK AP_HAL::dump_stack_trace()
 {
     // stack dump not available on this platform
 }
+void WEAK AP_HAL::dump_core_file()
+{
+    // core dump not available on this platform
+}

libraries/AP_HAL/system.h

@@ -25,5 +25,6 @@ uint64_t native_micros64();
 uint64_t native_millis64();
 
 void dump_stack_trace();
+void dump_core_file();
 
 } // namespace AP_HAL