Vector.Sum(Vector<T>) API implementation for horizontal add. (#53527)

Vector.Sum(Vector<T>) API implementation for horizontal add. (#53527)

* Vector.Sum(Vector<T>) API implementation for horizontal add.

* Fixed inccorrect referece to Arm64 AddAccross intrinsic function.

* Added implementation for hardware accelerated Vector<T>.Sum for long, ulong, float, double on ARM64.

* Fixed formatting issue.

* Correctness.

* Fixed compiler error for ARM64.

* Formatting issue.

* More explicit switch statement. Fixed wrong simd size for NI_Vector64_ToScalar.

* Fixed auto formatting issue.

* Use AddPairwiseScalar for double, long and ulong on ARM64 for VectorT128_Sum.

* Forgot ToScalar call after AddPairwiseScalar.

* Fixed wrong return type.