onnxruntime

increase cl mpcount since Compilation is moved on CPU machine (#20116)

### Description
The CPU machine has 16 cores, so we can increase the parallel count.
Compared with 2 runs.
1.
https://dev.azure.com/aiinfra/Lotus/_build/results?buildId=432328&view=results
2.
https://dev.azure.com/aiinfra/Lotus/_build/results?buildId=432331&view=results
The compilation took about 25 minutes if the parallel count is 15, while
it took 41 minutes if the parallel count is 3


### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
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Co-authored-by: Yi Zhang <your@email.com>