[META] Better X86 CPU targeting

[LebedevRI]

This is mostly a meta-issue to track sub-issues.

• Add enum with the X86 cpu's that are supported by halide
  • AMD CPU's added (-mtune=/-mcpu= support for x86 AMD CPU's #6655)
  • Intel CPU's are missing
• Support specifying the particular CPU for which the LLVM should tune for (-mtune=/-mcpu= support for x86 AMD CPU's #6655)
• Support auto-detecting the current host CPU
  • Base framework and proof-of-concept support for a single CPU (-mtune=native CPU autodetection for AMD Zen 3 CPU #6648)
  • Detection for other supported AMD CPU's is missing (get_amd_processor(): implement detection for the rest of supported AMD CPU's #6711)
  • Intel CPU's are missing
• Run-time multi-version selection support for tune CPU
  • Support encoding the tune cpu for the purposes of multi-versioning
  • Host CPU auto-detection (ideally, don't duplicate the code to do that)
  • Exact match
  • Fallback in case of no exact match (???)
• Support targeting the CPU (LLVM tune+halide features) by just the CPU name, similarly to -march=<...> (!!!)
• Support precisely targeting the CPU (LLVM target-cpu + LLVM target-features + halide features) by just the CPU name, exactly what -march=<...> does (!!!!!!)
  • Support encoding the target cpu for the purposes of multi-versioning
  • Exact match
  • Fallback in case of no exact match (???)