Fix xnnpack pybind (#2802)

Summary:
Fixing XNNPACK Pybind.

Currently pybindings for xnnpack fail due to uninitialized error. We notice the following:
```
(gdb) info symbol xnn_initialize
xnn_initialize in section .text of /home/maxren/.conda/envs/executorch/lib/python3.10/site-packages/torch/lib/libtorch_cpu.so
(gdb) info symbol xnn_create_subgraph
xnn_create_subgraph in section .text of /home/maxren/.conda/envs/executorch/lib/python3.10/site-packages/executorch/extension/pybindings/portable_lib.cpython-310-x86_64-linux-gnu.so
```

That is xnn_initialize symbole comes from libtorch_cpu, but when we call xnn_create_subgraph we call it from another library. It seems like explicitly adding XNNPACK to the portable_libs pybind fixes this problem.

Additionaly, we fix some fPIC issues when building XNNPACK.

Pull Request resolved: https://github.com/pytorch/executorch/pull/2802

Reviewed By: larryliu0820

Differential Revision: D55646601

Pulled By: mcr229

fbshipit-source-id: cf18de3246c9fdde4c08326c400d19cd407720e1