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

sys/conf/kern.post.mk

@@ -230,6 +230,7 @@ SRCS=	assym.inc offset.inc vnode_if.h ${BEFORE_DEPEND} ${CFILES} \
 	${SYSTEM_CFILES} ${GEN_CFILES} ${SFILES} \
 	${MFILES:T:S/.m$/.h/}
 DEPENDOBJS+=	${SYSTEM_OBJS} genassym.o genoffset.o genoffset_test.o
+DEPENDOBJS+=	${CLEAN:M*.o}
 DEPENDFILES=	${DEPENDOBJS:O:u:C/^/.depend./}
 .if ${MAKE_VERSION} < 20160220
 DEPEND_MP?=	-MP