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Improve feature detect for combined aarch64 features #1527

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Commits on Feb 16, 2024

  1. Improve feature detect for combined aarch64 features

    LLVM's `ssbs` and `mte` target_features represent two Arm features.
    Linux's HWCAP also represents the same two features, so this is just a
    documentation update.
    
    LLVM's `ras` target_feature represents two Arm features - FEAT_RAS and FEAT_RASv1p1. There is no runtime detection for this, so this is a no-op in stdarch.
    
    LLVM's `aes` feature covers both `FEAT_AES` and `FEAT_PMULL`, but Linux
    exposes seperate feature bits. This patch makes the `aes` target_feature
    correctly shortcut runtime `pmull` detection and also makes the `aes`
    feature check for `pmull` at runtime to bring it in line with the
    target_feature behaviour. In practice I think this makes the two runtime
    features identical since the ID_AA64ISAR0_EL1 register does not allow
    for PMULL without AES.
    adamgemmell authored and Amanieu committed Feb 16, 2024
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