Warn when `--iree-llvmcpu-target-cpu` defaults to "generic".

[ScottTodd]

Progress on https://github.com/iree-org/iree/issues/18561.

We have these high level flags (and MLIR attributes) controlling what code the llvm-cpu compiler target generates using LLVM:

• --iree-llvmcpu-target-triple
  • e.g. x86_64-pc-windows-msvc
  • See https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/TargetParser/Triple.h
• --iree-llvmcpu-target-cpu
  • e.g. znver2
• --iree-llvmcpu-target-cpu-features
  • e.g. +prfchw,-cldemote,+avx,+aes,+sahf,+pclmul,-xop,...

Some targets (x86 and riscv64) in LLVM can infer the "features" for a given "cpu" with the getFeaturesForCPU() functions. Other targets may be able to look up available features on the host via llvm::sys::getHostCPUFeatures(). Short of either of those, we must fall back to having users explicitly list the features they want the compiler to use.

If the target CPU and target CPU features are both omitted, we currently fall back to "generic", meaning no "features". This PR sends more warning information to stderr if that case is detected. We could go a step further and make the default case propagate an error through the compiler, requiring users to explicit set "host", "generic", a known cpu type, or a known list of features.

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TODO

• [ ] Update docs (https://iree.dev/guides/deployment-configurations/cpu/, source: docs/website/docs/guides/deployment-configurations/cpu.md)
• [ ] Audit the repository and other projects for usage of --iree-hal-target-backends=llvm-cpu that is missing an explicit --iree-llvmcpu-target-cpu or --iree-llvmcpu-target-cpu-features
• [ ] Test the different code paths (manually or with lit tests / automation)
• [ ] Add more context to error messages and docs, including an enumeration of accepted values for each flag

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Debugging tips while working on this:

• Run with --debug-only=iree-llvm-cpu-target to see the computed LLVMTarget get logged
• Note that multiple code paths run this code, including CLI flags that build defaultOptions_ for getDefaultExecutableTargets() and getVariantTarget/loadFromConfigAttr that may come from already constructed IR. We want error handling for all of them.