Split out vfp_new_thread

To keep the vfp thread creation code in one place move into vfp.c. This
will also help with adding SVE support as it depends on VFP.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35615

(cherry picked from commit baf8f20a4af43283a1fbee49b0c2ad906074f20f)