Arm64/Sve: Some misc items about SVE Vector Length

[kunalspathak]

List of various items related to vector length and VectorT handling in general for SVE:

• Enable Vector256 for SVE, when applicable. Details: https://github.com/dotnet/runtime/pull/101295/files#r1572607279
• See if we can enable the block copy, etc. using Vector256
• Fix getVectorTByteLength() to give correct current vector length instead of hard-coded to return 128.
• Make VectorT methods map to Sve intrinsics, whenever possible. So VectorT a+b should map to Sve.Add(a,b), etc.
• Reuse DOTNET_MaxVectorTBitWidth for Sve. WIP: Add option to change SVE vector length for current and children processes #101295

[kunalspathak]

@dotnet/arm64-contrib

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-runtime-intrinsics
See info in area-owners.md if you want to be subscribed.