[WIP] Recipe xnnpack

[hmaarrfk]

This may help with pytorch building. But I just want to use the CIs to test the package building process.

xref: conda-forge/pytorch-cpu-feedstock#60

Dependencies to unwind

• CLOG https://github.com/google/XNNPACK/blob/master/CMakeLists.txt#L91
• CPUINFO https://github.com/google/XNNPACK/blob/master/CMakeLists.txt#L101
• FP16 https://github.com/google/XNNPACK/blob/master/CMakeLists.txt#L111
• FXDIV https://github.com/google/XNNPACK/blob/master/CMakeLists.txt#L121
• PTHREADPOOL https://github.com/google/XNNPACK/blob/master/CMakeLists.txt#L131

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[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipes/xnnpack) and found it was in an excellent condition.

[hmaarrfk]

Need to update OSX:
pytorch/pytorch#57167 (comment)