Improve CPU provider build time with Eigen PCH (#32616)
### Description
Add `core/util/math_cpuonly.h` to the existing MSVC precompiled header
for the CPU providers target. This allows Eigen headers and templates
used by the provider translation units to be parsed once and reused.
### Build performance
The vcperf baseline identified `math_cpuonly.h` as the most expensive
included file for `onnxruntime_providers.vcxproj`:
- WCTR: 35.205 seconds
- Include count: 107
After the change, `math_cpuonly.h` no longer appeared in the top 15
included-file hotspots.
Five uninstrumented full CPU Release clean rebuilds were measured before
and after the change with MSVC and `--parallel 8`:
| Variant | Mean (s) | StdDev (s) |
|---|---:|---:|
| Baseline | 2519.522 | 136.238 |
| With PCH change | 2363.976 | 187.319 |
The average clean rebuild improved by 155.546 seconds (6.17%).
### Validation
- Two full CPU Release vcperf trace builds completed successfully.
- Five baseline and five post-change full CPU Release clean rebuilds
completed successfully.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
