arm64: Add the SVE registers to the signal frame

[andrew]

Summary

Use the new extended register support in the arm64 signal frame to
handle the SVE registers.

As these registers alias the VFP registers we use the floating point
register values to restore the lower bits of the SVE registers. This
is to support software that doesn't understand SVE to continue working.

Sponsored by: Arm Ltd

[imp]

I have the same question here I did about the other change to sigreturn: why no changes to setcontext and how does this interact with them?

[andrew]

I don't think we need to store the SCE registers in getcontextx. It's a function call that doesn't take an SVE register as an argument so the ABI doesn't require the upper bits of SVE registers to be preserved. We could restore the SVE registers in setcontextx if they are in the list of registers.

[andrew]

Rebase and check the SVE registers are only set once