arm: Fix initialization of VFP context

Make sure that pcb_vfpsaved is always initialized.
Create a vfp_new_thread helper that is heavily based on the arm64 logic.
While here remove un unnecessary assigment and add an assertion
to make sure that it's been properly initialized before we return
from a VFP exception.

Reported by: Mark Millard <marklmi@yahoo.com>
Tested by: Mark Millard <marklmi@yahoo.com>
Differential Revision: https://reviews.freebsd.org/D38698