kbuild: add cmd_and_savecmd macro

Separate out the command execution part of if_changed, as we did
for if_changed_dep.

This allows us to reuse it in if_changed_rule.

define rule_foo
$(call cmd_and_savecmd,foo)
$(call cmd,bar)
endef

Signed-off-by: Masahiro Yamada <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Tested-by: Nathan Chancellor <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>
Tested-by: Sedat Dilek <[e-mail]> # LLVM-14 (x86-64)

scripts/Kbuild.include

···
 if-changed-cond = $(newer-prereqs)$(cmd-check)$(check-FORCE)
 
 # Execute command if command has changed or prerequisite(s) are updated.
-if_changed = $(if $(if-changed-cond),                                        \
+if_changed = $(if $(if-changed-cond),$(cmd_and_savecmd),@:)
+
+cmd_and_savecmd =                                                            \
 	$(cmd);                                                              \
-	printf '%s\n' 'cmd_$@ := $(make-cmd)' > $(dot-target).cmd, @:)
+	printf '%s\n' 'cmd_$@ := $(make-cmd)' > $(dot-target).cmd
 
 # Execute the command and also postprocess generated .d dependencies file.
 if_changed_dep = $(if $(if-changed-cond),$(cmd_and_fixdep),@:)