AUTO_OBJ: Fix 'old style' kernel builds using wrong .OBJDIR.

There's no way currently to automatically prevent the bad .OBJDIR from being
created but it can at least be prevented from being used. Passing
WITHOUT_AUTO_OBJ=yes or MK_AUTO_OBJ=no or -DNO_OBJ in will prevent it.

Reported by: jeffr
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12989