Add xplat cxx_binary target for bench_sdpa

[kimishpatel]

Stack from ghstack (oldest at bottom):

• -> Add xplat cxx_binary target for bench_sdpa #18705
• Use unfused SDPA for short sequences (q_len <= 128 or kv_len <= 128) #18651
• Limit softmax to causally-valid elements in cpu_sdpa #18650
• Add quantized input support to cpu_sdpa #18649
• [Executorch] Add non-flash SDPA for decode #18648
• Add ONNX Runtime GQA-style SDPA benchmark #18647
• Add GEMM-based standard SDPA benchmark #18646
• Fix SLEEF preprocessor macro name to match ATen vec headers #18645

Add non_fbcode_target cxx_binary so bench_sdpa can be built via the
xplat build path (e.g. for on-device testing). Wrap the existing
cpp_benchmark in fbcode_target to avoid name collision between the
fbcode benchmark target and the xplat binary target.

Differential Revision: D96044315

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18705

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⏳ 12 Pending, 3 Unrelated Failures

As of commit 0d7815a with merge base 1debeb6 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-lora-multimethod-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Test CoreML Backend / test-coreml / test-backend-macos (coreml, models) / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test CoreML Backend / test-coreml / test-backend-macos (coreml, operators) / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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[kimishpatel]

submitted by accident, not meant to land immedidately