[Codegen][CPU] Seed C-bitcode ukernel framework with bf16 and i8 seeds. (#24567)

Adds compiler/plugins/target/LLVMCPU/builtins/ukernel/, the new home for
LLVMCPU-only C microkernels. Modelled after the ROCM C ukernel framework
(compiler/plugins/target/ROCM/builtins/ukernel/): compile each ukernel
to LLVM bitcode, embed those .bc files into iree-compile, and emit them
as self-contained `hal.executable_object`s on dispatch variants —
distinct from the legacy mmt4d ukernels under runtime/, which serve a
now-defunct VMVX path and are built twice.

This first commit lands the scaffolding plus two inner-K-loop seeds with
stub bodies (filled in by a later commit):
* `iree_uk_mma_x86_avx512bf16_1x16x2_f32_bf16.c` (bf16 x bf16 -> f32)
and `iree_uk_mma_x86_avx512vnni_16x16x2_i32_i8_casti16.c` (i8 x i8 ->
i32 via VNNI), plus `common.h` (no-stdlib stdint replacements);
* `iree_bitcode_library` + `iree_c_embed_data` rules, driven by a
`(stem, features, copts)` table so adding a seed is one row (CMake
regenerated; `iree_bitcode_library` in `bazel_to_cmake_converter.py`
extended to forward `out`, so the embedded filename is
`<name>.<arch>_<features>.bc`);
* a `README.md` design doc covering scope, build, IR representation,
end-to-end flow, and authoring/test guidance;
* `#iree_cpu.ukernel_provider`, the CPU analogue of
`#rocm.ukernel_provider`, declared in `IREECPUAttrs.td`/`.cpp` with a
`createAndReplaceWithUkernelOp` that currently delegates to the default
`LowerBitcodeUKernelsPass` fallback. Specialized `inner_tiled` handling
(threading `intrinsics_{m,n,k}` and the outer K count) will land in a
follow-up alongside `SelectUKernels` and pipeline integration.

Tests:
* roundtrip lit test for the new `#iree_cpu.ukernel_provider` attribute;
* lit tests running `--iree-codegen-lower-bitcode-ukernels` on ops
carrying both `iree_codegen.ukernel = "..."` and a user-supplied
`hal.executable.objects` bitcode (one per seed), asserting they are
rewritten to `iree_codegen.ukernel.generic` with the bitcode preserved.

Progress towards https://github.com/iree-org/iree/issues/24574.

---------

Signed-off-by: Benoit Jacob <jacob.benoit.1@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>