Why is MLAS_MAXIMUM_THREAD_COUNT set to 16?

### Describe the issue

I am doing some testing on cpus that have way more than 16 cores. I am seeing that MLAS is setting the max thread number to 16. Is there any reasons behind it?

### To reproduce

onnxruntime/core/mlas/lib/mlasi.h:290

### Urgency

_No response_

### Platform

Linux

### OS Version

Centos

### ONNX Runtime Installation

Built from Source

### ONNX Runtime Version or Commit ID

1.16.0

### ONNX Runtime API

C++

### Architecture

ARM64

### Execution Provider

Default CPU

### Execution Provider Library Version

_No response_