Fix Reduced Op build with empty FP16 kernel function tables

[adrianlizarraga]

Description

• Fixes compilation error for "reduced operator" builds with no FP16 kernels and MLAS_F16VEC_INTRINSICS_SUPPORTED enabled.
• Fixes linker error for "reduced operator" builds with QNN EP by excluding QNN EP unit tests. QNN EP unit tests require CPU EP operator implementations to evaluate accuracy.

Motivation and Context

Need to be able to build a reduced operator build with QNN EP. See https://github.com/microsoft/onnxruntime/blob/main/docs/Reduced_Operator_Kernel_build.md

The following example operator config file causes a compilation error when either MLAS_F16VEC_INTRINSICS_SUPPORTED is defined or QNN EP is enabled.

# reduced_op_config.txt
ai.onnx;12;Add

python tools\ci_build\build.py --include_ops_by_config reduced_op_config.txt --config Debug --build_wheel --build_shared_lib --skip_tests --build_dir build --parallel --use_qnn --qnn_home '<QNN_ROOT_DIR>'