kernel-toolchain: Skip world _obj and _cleanobj phases.

There's no good reason to treewalk the entire tree removing old OBJDIRS
and creating new ones when 'includes', 'libraries', and 'everything' are
all skipped. The only shared directory in the existing toolchain target
and world is build-tools. So handle cleaning in it directly if needed
only for the directories it wants to build.

The extra _obj/_cleanobj walks came in the initial kernel-toolchain
addition in r128189.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon