lib/abort_.c: Add prototype to make coverage build happy.

lib/embed/abort_.c

 #include <py/runtime.h>
 
-void abort_(void) {
+NORETURN void abort_(void);
+
+NORETURN void abort_(void) {
     nlr_raise(mp_obj_new_exception_msg(&mp_type_RuntimeError, "abort() called"));
 }