Add a "make spotless" rule for environments with BUILD_OBJ_ROOT not equal

to BUILD_OBJ_SRC. This will save the config.status and mklib files, then
wipe out the BUILD_OBJ_ROOT, copy back config.status and mklib, and then
run config.status to regenerate the makefiles. This target gives you a
completely clean/fresh BUILD_OBJ_ROOT.

llvm-svn: 18981