Remove `-fpermissive` flag from compilation patterns

Today, I ran into some problem, which turned out to be caused by the compiler happily passing an `int` to a function that expected a pointer. Closer investigation showed that a warning was issued, where I would have expected an error. This seems to because of the `-fpermissive` flag, which gets passed by the avr core by default. According to the gcc manpage:

```
-fpermissive
Downgrade some diagnostics about nonconformant code from
errors to warnings. Thus, using -fpermissive allows some
nonconforming code to compile.
```

I'm not sure what changes this makes exactly and if we need it at all. If the other changes are similar to this one, I think it should be disabled at some point (but it will probably break existing code that isn't quite correct, but now happens to work)...

For reference, here's a simplified version of my problem:

```cpp
void func(Foo *ptr);

int main() {
func(1);
return 0;
}
```

(the real problem was more subtle, involving a struct with a conversion operator to int, which got silently converted to a pointer containing rubbish when I forgot the `&` operator when trying to pass the struct's address to a function).

### Additional context

#### Additional requests

- https://github.com/arduino/ArduinoCore-avr/issues/268#issuecomment-248200635
- https://github.com/arduino/ArduinoCore-avr/issues/268#issuecomment-254016077
- https://github.com/arduino/ArduinoCore-avr/issues/268#issuecomment-256914284
- https://github.com/arduino/Arduino/issues/10154
- https://github.com/arduino/Arduino/issues/10154#issuecomment-643845516
- https://github.com/arduino/Arduino/issues/11402
- https://github.com/arduino/Arduino/issues/11639
- https://forum.arduino.cc/t/weird-warning-why/1420379

#### Related

- https://forum.arduino.cc/t/function-prototype-with-newline-compiler-error/1388103/13

---

Keywords

- "permissive"

Start by locating the avr core's compilation patterns that add `-fpermissive`, then reproduce the simplified invalid pointer-call example with and without the flag. Review the linked Arduino issues for compatibility concerns; done means the flag is removed where appropriate and the affected compilation checks or builds still pass.

Written by the indexing model from the issue text.