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
• Add Metal backend type definitions and utilities #15019
• Add Metal backend Python preprocessing, partitioning, and tests #15015
• [ET][Metal] Update aoti_common with additional AOTI functions needed by Metal backend #15003

[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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❌ 3 New Failures, 3 Pending

As of commit 57d9e45 with merge base 896178e ():

NEW FAILURES - The following jobs have failed:

• pull / test-qnn-wheel-packages-linux (3.10) / linux-job (gh)
  RuntimeError: Command docker exec -t ac089e4c1b838d942fdbc5879fc104eee304fea2438c6c3b1bbb37053630716e /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.11) / linux-job (gh)
  RuntimeError: Command docker exec -t 031dd9e6e727ba85af817967caf95e5cfb0b1fc80ff1d400e765da0a4ba2eece /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.12) / linux-job (gh)
  RuntimeError: Command docker exec -t f068f08465dce6224a2b04f0c0d1b0603816f93b409a88cb49a52f5173d6148d /exec failed with exit code 1

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