[Python] pyarrow does not disable SIMD CPU optimizations when set to do so.

### Describe the bug, including details regarding any error messages, version, and platform.

pyarrow exits immediately on my server with illegal instruction after running
python
import pyarrow

It does this even when setting the environment variable ARROW_USER_SIMD_LEVEL to NONE.

This problem has been reported by several users and occurs on a few of my systems.

### Component(s)

Python

Start by reproducing the reported `python` followed by `import pyarrow` failure with `ARROW_USER_SIMD_LEVEL=NONE` on an affected system. Trace how the Python component reads that setting and how SIMD initialization responds; done means importing pyarrow no longer exits with an illegal-instruction error when SIMD is disabled.

Written by the indexing model from the issue text.