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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@18981 91177308-0d34-0410-b5e6-96231b3b80d8