Add Metal backend build system and runtime integration

[manuelcandales]

Build system changes:

• Add EXECUTORCH_BUILD_METAL build option in default.cmake
• Fix AOTI linker flags for Apple platforms (use -export_dynamic)
• Extend AOTI build support to Metal backend
• Register Metal backend and configure Metal framework linkage
• Add PyTorch AOTI headers and OpenMP library detection with rpath

Runtime implementation:

• Add main Metal backend runtime providing entry point for AOTI-compiled
  model execution on Metal devices

This commit ties together all Metal backend components and enables
building the complete backend.

[manuelcandales]

Stack from ghstack (oldest at bottom):

• -> Add Metal backend build system and runtime integration #15024
• Metal backend: Add operator implementations #15023
• Metal backend: Implement the AOTI MPS shim #15022
• Metal backend: Add AOTI shims for memory management #15021
• Add Metal backend core ETMetal runtime. #15020

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15024

• 📄 Preview Python docs built from this PR

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❌ 1 New Failure, 3 Cancelled Jobs, 12 Unrelated Failures

As of commit 735eff5 with merge base 6e0c9f6 ():

NEW FAILURE - The following job has failed:

• Lint / link-check / lint-urls / linux-job (gh)

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / android / build-llm-demo / linux-job (gh)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-models-linux (add, portable, linux.2xlarge) / linux-job (gh)
• pull / test-models-linux-basic (mv3, xnnpack-quantization-delegation, cmake, linux.2xlarge, executorch-u... / linux-job (gh)

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• periodic / test-models-linux (buck2, mv3, portable, linux.2xlarge, 90) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-llama-runner-linux (fp32, xnnpack+custom+quantize_kv, linux.2xlarge, executorch-ubuntu-22.04... / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-llama-runner-linux (fp32, xnnpack+quantize_kv, linux.2xlarge, executorch-ubuntu-22.04-clang12) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-models-linux (ic3, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-models-linux (resnet18, portable, linux.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-models-linux-basic (mv3, portable, buck2, linux.2xlarge, executorch-ubuntu-22.04-clang12) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-models-linux-basic (mv3, xnnpack-quantization-delegation, buck2, linux.2xlarge, executorch-u... / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-models-linux-basic (vit, portable, buck2, linux.2xlarge, executorch-ubuntu-22.04-clang12) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-models-linux-basic (vit, portable, cmake, linux.2xlarge, executorch-ubuntu-22.04-clang12) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-samsung-models-linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-static-llama-qnn-linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / unittest-nxp-neutron / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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