cxxrtl: provide a way to perform unobtrusive power-on reset.

Although it is always possible to destroy and recreate the design to
simulate a power-on reset, this has two drawbacks:
  * Black boxes are also destroyed and recreated, which causes them
    to reacquire their resources, which might be costly and/or erase
    important state.
  * Pointers into the design are invalidated and have to be acquired
    again, which is costly and might be very inconvenient if they are
    captured elsewhere (especially through the C API).

backends/cxxrtl/cxxrtl_capi.cc

@@ -43,6 +43,10 @@ void cxxrtl_destroy(cxxrtl_handle handle) {
 	delete handle;
 }
 
+void cxxrtl_reset(cxxrtl_handle handle) {
+	handle->module->reset();
+}
+
 int cxxrtl_eval(cxxrtl_handle handle) {
 	return handle->module->eval();
 }