arm64: Add the SVE registers to the signal frame

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
Differential Revision: https://reviews.freebsd.org/D43307