AArch64 target feature cfgs are being incorrectly enabled

[Amanieu]

This seems to have been introduced in nightly 2022-03-16 which includes #90621.

Previous correct behavior:

# rustc +nightly-2022-03-15 --print cfg --target aarch64-unknown-linux-gnu | grep target_feature
target_feature="fp"
target_feature="llvm14-builtins-abi"
target_feature="neon"
target_feature="pmuv3"

New incorrect behavior:

# rustc +nightly-2022-03-16 --print cfg --target aarch64-unknown-linux-gnu | grep target_feature
target_feature="f32mm"
target_feature="f64mm"
target_feature="fhm"
target_feature="fp"
target_feature="fp16"
target_feature="jsconv"
target_feature="llvm14-builtins-abi"
target_feature="neon"
target_feature="pmuv3"
target_feature="sve"
target_feature="sve2"
target_feature="sve2-aes"
target_feature="sve2-bitperm"
target_feature="sve2-sha3"
target_feature="sve2-sm4"

cc @adamgemmell

[workingjubilee]

Commenting out the match arm in to_llvm_features for a given mapping alters the emitted features:

        // Rust ties fp and neon together. In LLVM neon implicitly enables fp,
        // but we manually enable neon when a feature only implicitly enables fp
        ("aarch64", "f32mm") => smallvec!["f32mm", "neon"],
        // ("aarch64", "f64mm") => smallvec!["f64mm", "neon"],
        ("aarch64", "fhm") => smallvec!["fp16fml", "neon"],
        // ("aarch64", "fp16") => smallvec!["fullfp16", "neon"],
        ("aarch64", "jsconv") => smallvec!["jsconv", "neon"],
        // ("aarch64", "sve") => smallvec!["sve", "neon"],
        ("aarch64", "sve2") => smallvec!["sve2", "neon"],
        // ("aarch64", "sve2-aes") => smallvec!["sve2-aes", "neon"],
        ("aarch64", "sve2-sm4") => smallvec!["sve2-sm4", "neon"],
        // ("aarch64", "sve2-sha3") => smallvec!["sve2-sha3", "neon"],
        ("aarch64", "sve2-bitperm") => smallvec!["sve2-bitperm", "neon"],
        (_, s) => smallvec![s],

Result is:

$ rustc +stage1 --print cfg --target aarch64-unknown-linux-gnu | grep target_feature
'+fp16' is not a recognized feature for this target (ignoring feature)
'+fp16' is not a recognized feature for this target (ignoring feature)
target_feature="f32mm"
target_feature="fhm"
target_feature="fp16"
target_feature="jsconv"
target_feature="llvm14-builtins-abi"
target_feature="neon"
target_feature="pmuv3"
target_feature="sve2"
target_feature="sve2-bitperm"
target_feature="sve2-sm4"

[pthariensflame]

@workingjubilee Should I open a separate issue for the pmuv3 part?

[workingjubilee]

That seems to have been a preexisting feature but yes, if you believe it is in error then feel free to open a new issue about it.

EDIT: Ah, yeah, I think our target for darwin isn't "an M1", in actuality, and that's what's causing conflict.

[pthariensflame]

@workingjubilee Alright; I've opened #95319.