No longer builds on avr targets

[RalfJung]

CI over at https://github.com/RalfJung/rustc-build-sysroot is failing with a build error in compiler-builtins:

error[E0432]: unresolved import `core::sync::atomic::AtomicU32`
 --> /Users/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/compiler_builtins-0.1.157/src/math/libm_math/support/feature_detect.rs:3:26
  |
3 | use core::sync::atomic::{AtomicU32, Ordering};
  |                          ^^^^^^^^^
  |                          |
  |                          no `AtomicU32` in `sync::atomic`
  |                          help: a similar name exists in the module: `AtomicU8`

For more information about this error, try `rustc --explain E0432`.
warning: `compiler_builtins` (lib) generated 1 warning (1 duplicate)
error: could not compile `compiler_builtins` (lib) due to 1 previous error; 1 warning emitted

This occurs when building against a target with a custom JSON spec:

{
        "arch": "avr",
        "cpu": "atmega328p",
        "data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",
        "env": "",
        "executables": true,
        "linker": "avr-gcc",
        "linker-flavor": "gcc",
        "linker-is-gnu": true,
        "llvm-target": "avr-unknown-unknown",
        "no-compiler-rt": true,
        "os": "none",
        "position-independent-executables": false,
        "exe-suffix": ".elf",
        "eh-frame-header": false,
        "pre-link-args": {
          "gcc": ["-mmcu=atmega328p"]
        },
        "late-link-args": {
          "gcc": ["-lgcc"]
        },
        "target-c-int-width": "16",
        "target-endian": "little",
        "target-pointer-width": "16",
        "vendor": "unknown"
    }

That same spec worked fine before, so seems like probably something in compiler-builtins is just not compatible with this 16bit target any more?

[bjorn3]

Already fixed by #909

[RalfJung]

Ah, that just didn't make it into the latest nightly yet.