lib32: Fix lib/libpmc/pmu-events files ending up in source directory.

This could happen with either WITHOUT_AUTO_OBJ=yes or MAKELEVEL>0 for
the initial 'make buildworld' command.

This now ensures that build-tools targets have 'make obj' ran if needed.

This is especially problematic for pmu-events since it is not directly
connected in the build. Normally the 'make includes' call right before
this implicitly creates the objdir with a 'make obj' already but
misses pmu-events because it is disconnected from lib/libpmc. Fixing that
would make this new 'make obj' pointless but it is being added to avoid
this problem in the future should another tool be connected like this.

Reported by: rgrimes, kib, kevans
Sponsored by: Dell EMC