build cadence hifi flow as a stand-alone cmake dependency (#5551)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5551

This PR enables
- stand-alone build of xtensa hifi kernels and runtime flow by build_cadence_xtensa.sh
- ported new slice-utils for slice op
- updated the xtensa build bash instruction
- removed unused hifi kernel header

Next pr will enable
- stand-alone build of cpu reference kernels and runtime flow by build_cadence_runner.sh

Reviewed By: hsharma35, mcremon-meta

Differential Revision: D63281705

fbshipit-source-id: b01914c6d147e87aca45f8158c87fa703187fd9e