Fix .depend.foo.o tracking for sys/conf/files defined compilations.

Some example files:

ia32_genassym.o
acpi_wakecode.o

The old mkdep method also lacked tracking these files.

Objects defined in sys/conf/files with no-obj and no-implicit-rule get their
own targets defined in the kernel Makefile but lack having their objects added
to DEPENDOBJS so never get a .depend file generated. Normally if an object is
in OBJS it will get a .depend file.

Fix this by looking for .o files in CLEAN and ensuring they are part of
the -MD filtering and .depend loading. This is a hack. Other solutions
could exist involving sys/conf/files or config(8) to auto add these to
DEPENDFILES/DEPENDOBJS but this method seems reliable enough without being
intrusive or error-prone for new files.

Reported by: bde
MFC after: 3 weeks
Sponsored by: Dell EMC