Update XNNPACK revision to fcbf55a (#2223)

Summary:
Update the revision of the XNNPACK library dependency to fcbf55a. Note that this is part of a synchronized update of XNNPACK for ExecuTorch, PyTorch, and Meta internal.

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Pull Request resolved: https://github.com/pytorch/executorch/pull/2223

Test Plan: Tested via internal and external CI for PyTorch and ExecuTorch. Also note that the full changeset was tested via D54463495.

Reviewed By: mcr229

Differential Revision: D54499911

Pulled By: GregoryComer

fbshipit-source-id: b9e2d3bbb44e63f7c3a422c215b8a7915adce46d