crunchgen: fix MK_AUTO_OBJ logic after r364166

r364166 converted echo -n /bin/pwd to a raw pwd invocation, leaving a
trailing newline at the end of path. This caused a later stat() of it to
erroneously fail and the fallback to MK_AUTO_OBJ=no logic proceeded as
unexpected.

Harry Schmalzbauer bissected the resulting build failure he experienced
(stable/12 host, -HEAD build) down to r365887. This change is mostly
unrelated, except it switches the build to bootstrapped crunchgen - clue!

I then bissected recent crunchgen changes going back a bit since we wouldn't
observe the failure immediately with -CURRENT in most configurations, which
landed me on r364166. After many intense head-scratching minutes and printf
debugging, I realized that the newline was the difference. This is where our
tale ends.

Reported by: Harry Schmalzbauer, O. Hartmann, Mike Tancsa, kevans
MFC after: 3 days