Arm64-SVE: Add scheme for scalable SVE state in the CONTEXT record

[snickolls-arm]

Replaces the fixed-length SVE state from the CONTEXT record with dynamically allocated space, similar to XSTATE, for saving and restoring vector/predicate register state, and the first-faulting register.

This reduces the size of the CONTEXT structure, as the SVE state is now optional. Subsystems that want to consume SVE state should set XSTATE_MASK_ARM64_SVE in CONTEXT::XStateFeaturesMask and allocate sufficient storage space for the trailer.

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[snickolls-arm]

@dotnet/arm64-contrib

I originally planned this work thinking that it was necessary to ensure SVE vector state is preserved across suspension, though I have learned that it's not while writing this patch, as the OS suspension context will do that already.

IIUC x64 saves its vector state in CONTEXT records with XSTATE, so I have updated ARM64's CONTEXT to have a similar view of SVE vector state. If anyone would be able to help me understand where this state might be used, that could be useful for future work.

I have tried to keep this data structure as simple as possible so it's easy to port to XSTATE format later on.

[dhartglassMSFT]

@janvorli would you be an appropriate person to take a look here? pal/CONTEXT changes