Update on "Fix SLEEF preprocessor macro name to match ATen vec headers"

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<float>::exp() correctly dispatches to
Sleef_expf4_u10 on ARM, which is the intended behavior.

Differential Revision: [D96044314](https://our.internmc.facebook.com/intern/diff/D96044314/)

[ghstack-poisoned]

Author: kimishpatel