arm64: Split out a savectx version of vfp_save_state

Rather than try to detect when vfp_save_state is called by savectx use
a separate function that sets up the pcb as needed.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43304

(cherry picked from commit 12257233e8fd94ab24e1a84ad87126af2a7be33b)