XNNPACK Buck build appears not to enable KleidiAI/SME2 while CMake enables KleidiAI by default

[mansnils]

The CMake path explicitly enables KleidiAI for XNNPACK by default, but I do not see
equivalent enablement in the OSS Buck/Buck2 XNNPACK targets. In particular, in
backends/xnnpack/targets.bzl, the xnnpack_backend target has the
Kleidi flag commented out:

# Uncomment to enable using KleidiAI Kernels
# "-DENABLE_XNNPACK_KLEIDI"

executorch/backends/xnnpack/targets.bzl

Line 49 in 3707efa

# Uncomment to enable using KleidiAI Kernels

Is this intentional?

If CMake defaults KleidiAI to on, is there a reason Buck/Buck2 should not also
default-enable KleidiAI when the target/toolchain supports it?

cc @GregoryComer @digantdesai @cbilgin @freddan80 @per @zingo @oscarandersson8218 @Sebastian-Larsson @robell @rascani