Improve helpers.h

Use C11 standard _Noreturn instead of attributes (except, of course, MSVC)
Remove unused helpers
Avoid trapping in release builds, in favor of just unreachability
Improve shim when __builtin_* are not available

src/link/assign.c

@@ -439,7 +439,7 @@ void assign_AssignSections(void)
 			return;
 	}
 
-	trap_;
+	unreachable_();
 }
 
 void assign_Cleanup(void)