[llvm] [Support] Fix xxh3_128bits for Win32 builds after #95863 (PR #96931)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Thank you for the fix! For my education: is there a workflow I can monitor that would catch this regression?

IIUC there is no bot for 32-bit Windows. In addition, Windows 11 has removed 32-bit support.
Therefore, the condition `#if defined(_MSC_VER) && defined(_M_IX86)` makes the failure unlikely caught by build bots. I am actually curious how you noticed the breakage due to xxh3_128bits :)

https://github.com/llvm/llvm-project/pull/96931