[X86] Split Subtarget ISA / Security / Tuning Feature Flags Definitions. NFC

Our list of slow/fast tuning feature flags has become pretty extensive and is randomly interleaved with ISA and Security (Retpoline etc.) flags, not even based on when the ISAs/flags were introduced, making it tricky to locate them. Plus we started treating tuning flags separately some time ago, so this patch tries to group the flags to match.

I've left them mostly in the same order within each group - I'm happy to rearrange them further if there are specific ISA or Tuning flags that you think should be kept closer together.

Differential Revision: https://reviews.llvm.org/D107370