Fix newvers.sh on macOS 10.15

It appears that the macOS /bin/sh echo now defaults to -e and therefore the
#define VERSTR included newline characters instead of \n. This caused compiler
errors due to unterminated strings. Fix by using printf instead of echo.
A less fragile solution might be to bootstrap the in-tree /bin/sh but that
requires more changes.

Reviewed By: brooks
Differential Revision: https://reviews.freebsd.org/D24136