kbuild: Leave objtool binary around with 'make clean'

The difference between 'make clean' and 'make mrproper' is documented in
'make help' as:

clean - Remove most generated files but keep the config and
enough build support to build external modules
mrproper - Remove all generated files + config + various backup files

After commit 68b4fe32d737 ("kbuild: Add objtool to top-level clean
target"), running 'make clean' then attempting to build an external
module with the resulting build directory fails with

$ make ARCH=x86_64 O=build clean

$ make -C build M=... MO=...
...
/bin/sh: line 1: .../build/tools/objtool/objtool: No such file or directory

as 'make clean' removes the objtool binary.

Split the objtool clean target into mrproper and clean like Kbuild does
and remove all generated artifacts with 'make clean' except for the
objtool binary, which is removed with 'make mrproper'. To avoid a small
race when running the objtool clean target through both objtool_mrproper
and objtool_clean when running 'make mrproper', modify objtool's clean
up find command to avoid using find's '-delete' command by piping the
files into 'xargs rm -f' like the rest of Kbuild does.

Cc: [e-mail]
Fixes: 68b4fe32d737 ("kbuild: Add objtool to top-level clean target")
Reported-by: Michal Suchanek <[e-mail]>
Closes: https://lore.kernel.org/[e-mail]/
Reported-by: Rainer Fiebig <[e-mail]>
Closes: https://lore.kernel.org/[e-mail]/
Acked-by: Josh Poimboeuf <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>
Tested-by: Nicolas Schier <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Nathan Chancellor <[e-mail]>

Makefile

···
 	$(Q)$(MAKE) -sC $(srctree)/tools/bpf/resolve_btfids O=$(resolve_btfids_O) clean
 endif
 
-PHONY += objtool_clean
+PHONY += objtool_clean objtool_mrproper
 
 objtool_O = $(abspath $(objtree))/tools/objtool
 
-objtool_clean:
+objtool_clean objtool_mrproper:
 ifneq ($(wildcard $(objtool_O)),)
-	$(Q)$(MAKE) -sC $(abs_srctree)/tools/objtool O=$(objtool_O) srctree=$(abs_srctree) clean
+	$(Q)$(MAKE) -sC $(abs_srctree)/tools/objtool O=$(objtool_O) srctree=$(abs_srctree) $(patsubst objtool_%,%,$@)
 endif
 
 tools/: FORCE
···
 $(mrproper-dirs):
 	$(Q)$(MAKE) $(clean)=$(patsubst _mrproper_%,%,$@)
 
-mrproper: clean $(mrproper-dirs)
+mrproper: clean objtool_mrproper $(mrproper-dirs)
 	$(call cmd,rmfiles)
 	@find . $(RCS_FIND_IGNORE) \
 		\( -name '*.rmeta' \) \

tools/objtool/Makefile

···
 	$(Q)$(RM) -r -- $(LIBSUBCMD_OUTPUT)
 
 clean: $(LIBSUBCMD)-clean
-	$(call QUIET_CLEAN, objtool) $(RM) $(OBJTOOL)
-	$(Q)find $(OUTPUT) -name '*.o' -delete -o -name '\.*.cmd' -delete -o -name '\.*.d' -delete
+	$(Q)find $(OUTPUT) \( -name '*.o' -o -name '\.*.cmd' -o -name '\.*.d' \) -type f -print | xargs $(RM)
 	$(Q)$(RM) $(OUTPUT)arch/x86/lib/cpu-feature-names.c $(OUTPUT)fixdep
 	$(Q)$(RM) $(OUTPUT)arch/x86/lib/inat-tables.c $(OUTPUT)fixdep
 	$(Q)$(RM) -- $(OUTPUT)FEATURE-DUMP.objtool
 	$(Q)$(RM) -r -- $(OUTPUT)feature
 
+mrproper: clean
+	$(call QUIET_CLEAN, objtool) $(RM) $(OBJTOOL)
+
 FORCE:
 
-.PHONY: clean FORCE
+.PHONY: clean mrproper FORCE