libc: rename arm and i386 Ovfork.S to vfork.S

While this has been Ovfork.S forever on i386 it differs from other
syscalls that require wrappers for no obvious reason so fix that.

Reviewed by: kib
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42909

(cherry picked from commit 0ea469bcd548d29bbbc970325e4fa851d0e4c022)