Unconditionally build zdb and ztest with -DDEBUG

Illumos unconditionally builds zdb and ztest with -DDEBUG. This helps
catch bugs and eliminates the need for commits like
202619623022722f30c2ee49931a4fa6896421c7, which changed ASSERTs to
VERIFYs. The following files in the illumos tree show this:

usr/src/cmd/zdb/Makefile.com
usr/src/cmd/ztest/Makefile.com

Given the usefulness of having early failure in these tools, we should
do it too.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #4095