ggml : add native AVX512-FP16 support for F16 operations (llama/20529)

ggml : add native AVX512-FP16 support for F16 operations (llama/20529)

The overall benchmark speed remains almost the same because the CPU is
now calculating faster than the RAM can deliver the data. (See perf stat
results below showing 2.7 billion fewer instructions).

Also note that this path will be only enabled for native build or with
custom flags.

now:
```
 Performance counter stats for 'build/bin/llama-bench -m Qwen3-0.6B-f16.gguf -p 512 -n 128':

        189,073.52 msec task-clock                       #   14.658 CPUs utilized
               404      context-switches                 #    2.137 /sec
                19      cpu-migrations                   #    0.100 /sec
           372,390      page-faults                      #    1.970 K/sec
   310,877,195,595      instructions                     #    0.54  insn per cycle
   581,071,530,602      cycles                           #    3.073 GHz
    19,352,107,994      branches                         #  102.352 M/sec
        48,304,438      branch-misses                    #    0.25% of all branches
    84,998,431,152      L1-dcache-loads                  #  449.552 M/sec
    12,186,410,279      L1-dcache-load-misses            #   14.34% of all L1-dcache accesses

      12.899358742 seconds time elapsed

     187.823044000 seconds user
       1.253416000 seconds sys
```

before:
```
 Performance counter stats for 'build/bin/llama-bench -m Qwen3-0.6B-f16.gguf -p 512 -n 128':

        190,594.56 msec task-clock                       #   14.652 CPUs utilized
               436      context-switches                 #    2.288 /sec
                22      cpu-migrations                   #    0.115 /sec
           372,782      page-faults                      #    1.956 K/sec
   313,574,921,966      instructions                     #    0.54  insn per cycle
   586,064,970,425      cycles                           #    3.075 GHz
    19,585,778,563      branches                         #  102.761 M/sec
        48,437,488      branch-misses                    #    0.25% of all branches
    86,219,336,628      L1-dcache-loads                  #  452.370 M/sec
    12,232,085,771      L1-dcache-load-misses            #   14.19% of all L1-dcache accesses

      13.007923164 seconds time elapsed

     189.395316000 seconds user
       1.202612000 seconds sys
```

Signed-off-by: Adrien Gallouët <redacted>