Fix test 'make clean' target for empty $(DSYM) on FreeBSD

A freebsd issue with rm prevents '[g]make clean' from working if $(DSYM)
is empty (fts_open(3) fails if passed an empty argument).

To work around this, simplify the clean target by using three separate
rm invocations: one for the common files, one for the case of non-empty
$(DYLIB_NAME), and one for non-empty $(DSYM).

Issue diagnosed (and reported to FreeBSD) by John Wolfe.

llvm.org/pr17933