arm64: Create a version of vfp_save_state for cpu_switch

This will be used when we add SVE support to reduce the registers
needed to be saved on context switch.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43305

(cherry picked from commit a30149b2a9c6ac5280523eea9570e5b5e5f1fdf8)