Clean up aarch64 quantized GEMM dispatch (#16120)
### Description
- Add a new field to `MLAS_PLATFORM` for S8S8 GEMM dispatch.
- Set this field to either dot product instructions or NEON MLA in
platform.cpp.
- Clean up dispatch selector in qgemm.h.
### Motivation and Context
This will allow future extensibility as other functions that use other
ARM64
extensions for quantized matrix multiplication.
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Co-authored-by: Skand Hurkat <skhurkat@microsoft.com>
