ifdef gettimeofday_err so it only runs on NetBSD

NetBSD goes above and beyond the POSIX spec because it can run pointer
checks in kernel(-ish) space to determine whether or not a pointer is
valid with gettimeofday. FreeBSD and Linux don't do this, so only
compile/run the test on NetBSD.
Fix the build

Remove the unnecessary dependency on libatf-c in lib/libthr/tests/dso .
This didn't used to be a problem because libatf-c was not a private
library, but because it's now a private library, this case now appears
to fail because it's not setting the USEPRIVATELIB variable which sets
LDFLAGS appropriately.