Fix SLEEF preprocessor macro name to match ATen vec headers

[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

The ATen NEON vectorized math headers (vec128_float_neon.h) check for
AT_BUILD_ARM_VEC256_WITH_SLEEF to enable SLEEF intrinsics for exp(),
log(), etc. ExecuTorch's get_vec_preprocessor_flags() was defining
ET_BUILD_ARM_VEC256_WITH_SLEEF (wrong prefix), so the USE_SLEEF macro
always took the fallback path: map(std::exp) — scalar exp called
per-element with full vector load/store overhead wrapping it.

With this fix, Vectorized::exp() correctly dispatches to
Sleef_expf4_u10 on ARM, which is the intended behavior.

Differential Revision: D96044314

[pytorch-bot]

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❌ 1 New Failure, 1 Cancelled Job, 7 Unrelated Failures

As of commit 9f43d8d with merge base 1debeb6 ():

NEW FAILURE - The following job has failed:

• pull / test-models-linux-basic (vit, portable, cmake, linux.2xlarge, executorch-ubuntu-22.04-clang12) / linux-job (gh)
  RuntimeError: Command docker exec -t 2c6380c8c250ecebba432ac4d76a0e5f48febbf909d7b39fcc8038c492b1a334 /exec failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest / macos / macos-job (gh)

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

• pull / test-llama-runner-linux (bf16, custom, 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-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / 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 / windows / windows-job (gh) (matched win rule in flaky-rules.json)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (matched win rule in flaky-rules.json)
  ##[error]The operation was canceled.

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

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

• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• 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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