Fix MSVC x86 prefetch architecture check

[lpyu001]

Fix the MSVC 32-bit x86 architecture check used for prefetch support.

MSVC defines _M_IX86 for x86 targets, not _M_I86. As a result, the current condition doesn't enable the _mm_prefetch
implementation for 32-bit MSVC builds.

The same issue was recently fixed in CPython after its prefetch logic, which was derived from zstd, was found to contain the same typo:

python/cpython#157907

[meta-cla]

Hi @lpyu001!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!

[meta-cla]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!