AArch64: add support for newer Apple CPUs

They're roughly ARMv8.6. This works in the .td file, but in
AArch64TargetParser.def, marking them v8.6 brings in support for the SM4
cryptographic hash and we don't actually have that. So TargetParser side
they're marked as v8.5, with the extra features (BF16 and I8MM added manually).

Finally, A16 supports the HCX extension in addition to v8.6. This has no
TargetParser implications.