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

sys/conf/newvers.sh

@@ -343,7 +343,7 @@ EOF
 )
 vers_content_old=$(cat vers.c 2>/dev/null || true)
 if [ "$vers_content_new" != "$vers_content_old" ]; then
-	echo "$vers_content_new" > vers.c
+	printf "%s" "$vers_content_new" > vers.c
 fi
 
 echo $((v + 1)) > version