asm: Add function to get stack size

[LuigiPiucco]

Fix #175.

The check with u8::MAX is there because some MCUs (the really small ones whose data space address fits in a u8) may not have SPH. Because extern statics are not as optimizable as consts, the resulting binary can't eliminate the opposing branch, something that could be worked on.

[Rahix]

I was about to try testing this on real hardware, then I noticed we first have to move avr-hal to the new avr-device after #157 ... (For reference, I had already started on this in Rahix/avr-hal#545 a while ago)

I'll leave this PR open until that work has concluded, I don't want to merge it without evidence that it produces the expected results. Apart from that I think it's ready — thanks @LuigiPiucco for again picking up the work!

[Rahix]

Yay, managed to test this and it seems to work as expected! 🎉

For reference, I used the following code:

#![no_std]
#![no_main]

use arduino_hal::prelude::*;
use panic_halt as _;

#[inline(never)]
fn stack_grower(level: usize, serial: &mut impl ufmt::uWrite) -> usize {
    let stack_size = avr_device::asm::get_stack_size();

    let next_size = if level < 16 {
        stack_grower(level + 1, serial)
    } else {
        stack_size
    };

    let _ = ufmt::uwriteln!(
        serial,
        "Stack At Level {}: {} (diff {})\r",
        level,
        stack_size,
        next_size - stack_size
    );

    stack_size
}

#[arduino_hal::entry]
fn main() -> ! {
    let dp = arduino_hal::Peripherals::take().unwrap();
    let pins = arduino_hal::pins!(dp);
    let mut serial = arduino_hal::default_serial!(dp, pins, 57600);

    ufmt::uwriteln!(&mut serial, "Hello from Arduino!\r").unwrap_infallible();

    stack_grower(1, &mut serial);

    loop {}
}

which produces the following output

Hello from Arduino!
Stack At Level 16: 133 (diff 0)
Stack At Level 15: 125 (diff 8)
Stack At Level 14: 117 (diff 8)
Stack At Level 13: 109 (diff 8)
Stack At Level 12: 101 (diff 8)
Stack At Level 11: 93 (diff 8)
Stack At Level 10: 85 (diff 8)
Stack At Level 9: 77 (diff 8)
Stack At Level 8: 69 (diff 8)
Stack At Level 7: 61 (diff 8)
Stack At Level 6: 53 (diff 8)
Stack At Level 5: 45 (diff 8)
Stack At Level 4: 37 (diff 8)
Stack At Level 3: 29 (diff 8)
Stack At Level 2: 21 (diff 8)
Stack At Level 1: 13 (diff 8)