Use 16-byte alignment for CMSIS-NN scratch buffers

[rascani]

Summary

MVE vector operations require 16-byte aligned data. Replace alignof(int16_t) (2 bytes) for scratch buffer allocations with a shared kCortexMMveAlignment constant.

[pytorch-bot]

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❌ 1 Awaiting Approval, 2 New Failures

As of commit b6cb614 with merge base 1f4ad07 ():

AWAITING APPROVAL - The following workflow needs approval before CI can run:

• periodic (gh)

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 1690bb7b5ddeac5d3ce705199e556e128fdcfef0473cc6c4fa5c90c0c89e0827 /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t bb698aa3d8278311aae3b829d0bc5bf44ad5315b3ad6133be51255a8523c9790 /exec failed with exit code 1

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