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.

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