AArch64: switch Apple CPUs (that support it) to v8.6a. (#73497)

We pretended they were v8.5a in the past because LLVM's modelling used
to fold SM4 crypto support into v8.6a (which the CPUs don't actually
have). That's changed in the last year so we can use the real value.

This is mostly a tidy-up commit before one that'll bring in A17 and M3.