libc: don't needlessly add vfork.o to NOASM

For architectures where vfork.S was named Ovfork.S this was needed, but
it was always pointless here as an entry in either MDASM or NOASM is
equivalent.

Reviewed by: kib
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42914

(cherry picked from commit ec27c0bb3eea73be4db6cd2f275db6c516e12d00)