`-C target_cpu=cortex-a72` (and `-target-cpu=native` on Raspberry Pi) wrongly enables crypto features that are optional on Cortex-A72

Building with RUSTFLAGS="-C target_cpu=cortex-a72" statically enables the target_feature="aes", target_feature="crc", target_feature="pmuv3", and target_feature="sha2". However, at least the crypto features AES, CRC, and SHA2 are optional on this CPU. The definition for this target is wrong. See the upstream LLVM bug: https://github.com/llvm/llvm-project/issues/90365.

The main consequence is that crypto libraries that use cfg(target_feature = ...) feature detection for these hardware instructions are getting miscompiled, causing the programs to, at best, crash with an illegal instruction exception. This particular affects Raspberry Pi users compiling with RUSTLFAGS=-target-cpu=native. From https://github.com/briansmith/ring/issues/1858#issuecomment-2093675988:

$ rustc --print cfg --target=aarch64-unknown-linux-gnu -C target_cpu=cortex-a72 | grep feature
target_feature="aes"
target_feature="crc"
target_feature="neon"
target_feature="pmuv3"
target_feature="sha2"

Without -C target_cpu=cortex-a72 we get the correct feature flags:

$ rustc --print cfg --target=aarch64-unknown-linux-gnu | grep feature
target_feature="neon"

I verified this is an issue on Rust 1.61 stable, 1.78 stable, and rustc 1.80.0-nightly (6e1d94708 2024-05-10).

Although some crypto libraries may work around this issue, these workarounds have negative consequences. In the case of ring's workaround, the result of the workaround is bloat and worse performance on all AArch64 CPUs that actually are guaranteed to have the crypto extensions (except on Fuchsia, Windows, and macOS).

Reproduce the reported behavior with rustc --print cfg --target=aarch64-unknown-linux-gnu and -C target_cpu=cortex-a72, comparing the feature lists shown in the issue. Read the linked upstream LLVM bug and the target CPU feature definition involved. Done means optional AES, CRC, and SHA2 features are no longer enabled for Cortex-A72 unless supported.