Use dynamic CPU count for cmake --build -j in docs and test scripts

[ShamSaleem]

Summary

Several build docs and test/ scripts hardcode the cmake --build -j
parallelism (-j9, -j10), which assumes a fixed machine. This replaces
them with a portable expression that derives "core count + 1" at runtime —
nproc on Linux, sysctl -n hw.ncpu on macOS:

-j$(( $(nproc 2>/dev/null || sysctl -n hw.ncpu) + 1 ))

"core count + 1" matches the guidance already documented in
docs/source/using-executorch-building-from-source.md. The nproc → sysctl
fallback keeps the commands working on both Linux and macOS, and the
arithmetic degrades gracefully to -j1 if neither tool is available.

Partial fix for #10887. Scope is limited to general (non-vendor) docs and
contributor-facing test/ build scripts (9 files). Vendor-backend scripts
(cadence, vulkan, coreml, qualcomm, mediatek, samsung, mps, nxp), CI scripts
under .ci/, and non-cmake -j flags are intentionally left for follow-ups.

Test plan

• lintrunner passes on all changed files.
• bash -n passes on the three modified shell scripts.
• Verified the expression evaluates to a valid integer on Linux, e.g. 17
  on a 16-core machine.

[pytorch-bot]

🔗 Helpful Links

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⚠️ 15 Awaiting Approval

As of commit b340e88 with merge base 484f72c ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

• Build Aarch64 Linux Wheels (gh)
• Build documentation (gh)
• Build Linux Wheels (gh)
• Build macOS Wheels (gh)
• Build Presets (gh)
• Build Windows Wheels (gh)
• Lint (gh)
• periodic (gh)
• pull (gh)
• Test ARM Backend (gh)
• Test CoreML Backend (gh)
• Test QNN Backend (gh)
• Test Vulkan Backend (gh)
• Test WebGPU Backend (gh)
• Test XNNPACK Backend (gh)

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[linux-foundation-easycla]

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• ✅ login: ShamSaleem / name: ShamSaleem (b340e88)

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