AP_HAL: move dump_strack_trace into HAL

Use a weak definition to allow subclasses to override

libraries/AP_HAL/system.cpp

@@ -4,3 +4,8 @@ uint16_t WEAK AP_HAL::millis16()
 {
     return millis() & 0xFFFF;
 }
+
+void WEAK AP_HAL::dump_stack_trace()
+{
+    // stack dump not available on this platform
+}

libraries/AP_HAL/system.h

@@ -18,4 +18,6 @@ uint16_t millis16();
 uint64_t micros64();
 uint64_t millis64();
 
+void dump_stack_trace();
+
 } // namespace AP_HAL