Discard the arm64 VFP state before resetting it

When resetting the VFP state we need to discard any old state so we don't
try to save it on a context switch. Move this first so resetting the pcb
is safe to perform outside a critical section.

Reviewed by: arichardson
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D29401