[BugFix] Fix clang spinloop mwaitx include

[johnnyychiu]

Purpose

Fixes #45515.

csrc/spinloop.cpp included <mwaitxintrin.h> directly for the x86 MONITORX/MWAITX intrinsics. Clang rejects that header when included directly and asks callers to include <x86intrin.h> instead. Switching to the umbrella x86 intrinsic header keeps _mm_monitorx and _mm_mwaitx available while allowing clang-hosted builds to compile the spinloop target.

Test Plan

• Inspect the resulting diff to confirm only the x86 intrinsic include changed.
• Run git diff --check.

Test Result

• git diff --check passed.
• Not run locally: clang build or syntax check. This Windows workspace does not have clang++ on PATH, so I could not reproduce the ROCm clang compile locally.

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[njhill]

Thanks @johnnyychiu!