Add top-level CMake backends target

[GregoryComer]

Add a top level backends CMake target, which includes all configured backends. I validate this change by building a simple runner using the executorch_backends target and verifying that it was able to build and run an XNNPACK-delegated binary. Once the changes land in ExecuTorch, this will be long-term validated in executorch-examples CI.

To allow the executor runner target to use the executorch_backends target, I've also moved the Vulkan defs above the runner. This causes issues with the vulkan_executor_runner target, so I'm updating the main runner to include Vulkan and remove the Vulkan-specific one (subject to @SS-JIA's signoff).

This is done in the context of top-level CMake targets, tracked in #12293.

[GregoryComer]

Stack from ghstack (oldest at bottom):

• Update docs to use top-level CMake targets #12807
• Add top-level CMake kernels target #12806
• Add top-level CMake extensions target #12696
• -> Add top-level CMake backends target #12689

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12689

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[GregoryComer]

Vulkan build failure in the unittest job is real. Fixing that and then this should be good to land.