Avoid using non-portable dd status=none flag

Copying the approach chosen in r309412. This fixes building the libc tests
on a macOS host since the macOS /bin/dd binary does not support status=none.

As there only seem to be two uses, this commit changes the two Makefiles.
If this becomes more common, we could also add a wrapper bootstrap script
that ignores status= and forwards the remaining args to the real dd.
Another alternative would be to remove the status flag and pipe stderr to
/dev/null, but them we lose error messages.

Reviewed By: brooks
Differential Revision: https://reviews.freebsd.org/D24785