Arm64 SVE: Implement missing extract methods

[a74nh]

The following mask methods were approved in #98184 but are not yet implemented:

public static unsafe T ExtractAfterLastScalar(Vector<T> value); // LASTA // predicated
public static unsafe Vector<T> ExtractAfterLastVector(Vector<T> value); // LASTA // predicated
public static unsafe T ExtractLastScalar(Vector<T> value); // LASTB // predicated
public static unsafe Vector<T> ExtractLastVector(Vector<T> value); // LASTB // predicated

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[a74nh]

These APIs return either a scalar or a scalar in a Neon register. Therefore, it doesn't make sense that the mask has been hidden away for these, as they cannot be embedded inside a conditional select.

The APIs should be:

public static unsafe T ExtractAfterLastScalar(Vector<T> mask, Vector<T> value); // LASTA // predicated
public static unsafe Vector<T> ExtractAfterLastVector(Vector<T> mask, Vector<T> value); // LASTA // predicated
public static unsafe T ExtractLastScalar(Vector<T> mask, Vector<T> value); // LASTB // predicated
public static unsafe Vector<T> ExtractLastVector(Vector<T> mask, Vector<T> value); // LASTB // predicated

Note that the ConditionalExtract methods (CLASTA,CLASTB) do have the mask in the API.

We plan to implement these with the mask, and then raise an API review issue to cover the differences.