[Codegen][CPU] Register embedded ukernel bitcode + provider lookup. (#24568)

Wires the LLVMCPU plugin and the `#iree_cpu.ukernel_provider` attribute
to actually resolve ukernel bitcode:

* On plugin init, the embedded `iree_uk_cpu_bitcode` TOC is iterated and
each `.bc` is added to the global `EmbeddedDataDirectory` (mirrors
`addAMDGPUUkernelBitcodeToGlobalEmbeddedDataDirectory` in
`ROCMTarget.cpp`).

* `UKernelProviderAttr::createAndReplaceWithUkernelOp` now resolves the
bitcode by name: user-supplied `hal.executable.objects` ancestors first
(BYO and BYO-override paths), then the embedded directory. If found and
not already on the op, the attribute is attached to the source op; the
default `LowerBitcodeUKernelsPass` rewrite then preserves it on the
resulting `ukernel.generic`. The provider still returns `std::nullopt`
for the rewrite itself — specialized `inner_tiled` handling (scalar
`intrinsics_{m,n,k}` + outer K count + ACC inner stride) lands with
SelectUKernels in a follow-up.

* Filename match is `<ukernelName>.<features>.bc` (or bare
`<ukernelName>.bc`), matching the build rules under
`compiler/plugins/target/LLVMCPU/builtins/ukernel/`. Multi-variant
disambiguation by target feature set also lands with SelectUKernels.

BUILD.bazel: LLVMCPU plugin gains a dep on

`//compiler/plugins/target/LLVMCPU/builtins/ukernel:iree_uk_cpu_bitcode`,
and the `iree_cpu` dialect picks up HAL IR + `EmbeddedDataDirectory` so
the provider implementation can live with the attribute (consistent with
how `ROCMUkernelBitcodeSupport.cpp` sits inside the ROCM dialect).

Tests: a three-way lit test (`lookup_builtin_and_byo.mlir`) covers each
branch of the lookup:
1. Built-in only — no user objects; expect a `dense_resource<…>` with
the embedded bf16 bitcode (2732 bytes) attached.
2. BYO override — user supplies bitcode under the built-in's filename;
expect the user's bytes to flow through unchanged (and CHECK-NOT against
`dense_resource` to catch a regression where the provider silently swaps
in the embedded copy).
3. Pure BYO — user-only name with no matching built-in; expect the
user's bitcode to surface on the rewritten op.

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 4.7 <noreply@anthropic.com>