build: use bare (and portable) echo instead of echo -n

There's no need to use echo -n here. A single echo will do nicely.
This fixes the post-buildworld output on a macos build, where echo -n
is implemented like System V instead of BSD (so you get two lines
first one starting with -n).

Sponsored by: Netflix
Reviewed by: jrtc27, emaste
Differential Revision: https://reviews.freebsd.org/D42869