newvers.sh: avoid regenerating vers.c if content unchanged

When reproducible build mode is enabled vers.c may be unchanged between
successive builds. In this case avoid changing the file's metadata so
that it does not cause dependent targets to be rebuilt.

Sponsored by: The FreeBSD Foundation
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D17892

(cherry picked from commit 179460e1481ef98aa6824681ee472b0764c9d9a0)