ggml : add CPU backend reference implementation (wip)

[danbev]

This commit introduces a CPU reference implementation for GGML,
designed primarily for testing and validation purposes.

The motivation for this addition is to have a pure C CPU backend
implementation that does not use any hardware-specific optimizations
or intrinsics. This will allow for testing the CPU backend variants
against the reference implementation to ensure correctness

---

Building

$ cmake -B build \
    -DGGML_CPU_REF_BACKEND=ON
    -DGGML_BACKEND_DL=ON \
    -DGGML_CPU_ALL_VARIANTS=ON

Listing variants

$ ./build/bin/test-backend-ops cpu-variants --list
CPU variants:
  CPU-alderlake   - 12th Gen Intel(R) Core(TM) i7-1260P

Testing a variant

$ ./build/bin/test-backend-ops cpu-variants --variant CPU-alderlake -o ADD
Testing CPU variant 'CPU-alderlake' against cpu-ref backend...

  ADD(type=f16,ne=[1,1,8,1],nr=[1,1,1,1],nf=1): OK
  ADD(type=f16,ne=[1,1,1,1],nr=[32,1,1,1],nf=1): OK
  ADD(type=f16,ne=[1,1,320,320],nr=[1,1,1,1],nf=1): OK
  ...
  14471/14471 tests passed

[slaren]

There is no need to duplicate the CPU backend to do this - that would be a maintenance nightmare. You can add variant that is built in similar way that all the other CPU backend variants when using GGML_CPU_ALL_VARIANTS, just with all the instruction sets disabled so that only the C implementation is used.

[danbev]

@slaren Ah right, that does sound much better and simpler than this. I'll make the changes. Thanks!