interrupt: Support Xtensa

[taiki-e]

This adds portable_atomic_unsafe_assume_single_core support for Xtensa targets without atomic CAS.

[MabezDev]

Just swinging by to say thanks for this, will be very useful for the esp-rs community! The implementation looks good to me also, we have a similar implementation in esp-hal, just with a different level (to allow debug interrupts to occur).

I noticed some TODO's in the CI, if you are having issues setting up the toolchain let me know. We also have github action for installing the toolchain in CI if that's useful: https://github.com/esp-rs/xtensa-toolchain

[taiki-e]

I noticed some TODO's in the CI

TODO in CI is about tests on CI (using a simulator).

I mainly use QEMU and semihosting crate to test no-std targets, so I tried to do the same with Xtensa, but the compiler didn't seem to recognize the simcall instruction so I gave that up testing on CI and decided to test locally using your wokwi-server (it worked well).

Tests passed except for a few tests that ran into compiler bugs (?).

• wrong value on fetch_nand

  let a = portable_atomic::AtomicUsize::new(0x13);
  assert_eq!(a.fetch_nand(0x31, portable_atomic::Ordering::Relaxed), 0x13);
  assert_eq!(a.load(portable_atomic::Ordering::Relaxed), !(0x13 & 0x31));

  panicked at 'assertion failed: `(left == right)`
    left: `4290789375`,
   right: `4294967278`', src/main.rs:21:5
  

• compile failure on fetch_{min,max}

  let a = portable_atomic::AtomicUsize::new(23);
  assert_eq!(a.fetch_min(24, Ordering::Relaxed), 23);
  assert_eq!(a.load(Ordering::Relaxed), 23);

  error: Undefined temporary symbol .LBB1_2
  

(These may be known problems.)

[taiki-e]

Published in 1.3.0.