GHSA-2gqc-6j2q-83qp

### Summary

`thumbv6m-none-eabi` (Cortex M0, M0+ and M1) compiler emits non-constant time assembly when using `cmovnz` (portable version). I did not found any other target with the same behaviour but I did not go through all targets supported by Rust.

### Details

It seems that, [during `mask` computation](https://github.com/RustCrypto/utils/blob/9e555db060c80f4669d804f448a524a37d201b32/cmov/src/portable.rs#L78), an LLVM optimisation pass is detecting that [`bitnz`](https://github.com/RustCrypto/utils/blob/9e555db060c80f4669d804f448a524a37d201b32/cmov/src/portable.rs#L13) is returning 0 or 1, that can be interpreted as a boolean. This intermediate value is not masked by a call to `black_box` and thus the subsequent [`.wrapping_sub(1)`](https://github.com/RustCrypto/utils/blob/9e555db060c80f4669d804f448a524a37d201b32/cmov/src/portable.rs#L78C1-L78C84) can be interpreted as a conditional bitwise conditional not.

### PoC

This is an attempt at having a minimal faulty code. In a library crate with an up-to-date `cmov` as only dependency, the content of `src/lib.rs` is:

```rust #![no_std] use cmov::Cmov;

#[inline(never)] pub fn test_ct_cmov(a: &mut u8, b: u8, c: u8) { a.cmovnz(&b, c); } ```

The resulting assembly emitted (shown using `cargo asm --release --target thumbv6m-none-eabi` that uses [`cargo-show-asm`](https://crates.io/crates/cargo-show-asm)):

<details> <summary>Collapsed assembly</summary>

```asm .section .text.not_ct::test_ct_cmov,"ax",%progbits .globl not_ct::test_ct_cmov .p2align 1 .type not_ct::test_ct_cmov,%function .code 16 .thumb_func not_ct::test_ct_cmov: .fnstart .cfi_sections .debug_frame .cfi_startproc .save {r7, lr} push {r7, lr} .cfi_def_cfa_offset 8 .cfi_offset lr, -4 .cfi_offset r7, -8 .setfp r7, sp add r7, sp, #0 .cfi_def_cfa_register r7 .pad #8 sub sp, #8 movs r3, #0 lsls r2, r2, #24 bne .LBB0_2 mvns r3, r3 .LBB0_2: ldrb r2, [r0] str r3, [sp, #4] str r3, [sp] mov r3, sp @APP @NO_APP ldr r3, [sp] bics r1, r3 ands r2, r3 adds r1, r2, r1 strb r1, [r0] add sp, #8 pop {r7, pc} ```

</details>

The non-constant time assembly is:

```asm bne .LBB0_2 mvns r3, r3 .LBB0_2: ```

### Impact

The exact impact is unclear, especially since `cmov` clearly warns users that the portable version is best-effort.