kbuild: fix rebuild of generic syscall headers

Commit fbb5c060 ("kbuild: add syscall table generation to
scripts/Makefile.asm-headers") started to generate syscall headers
for architectures using generic syscalls.

However, these headers are always rebuilt using GNU Make 4.4.1 or newer.

When using GNU Make 4.4 or older, these headers are not rebuilt when the
command to generate them is changed, despite the use of the if_changed
macro.

scripts/Makefile.asm-headers now uses FORCE, but it is not marked as
.PHONY. To handle the command line change correctly, .*.cmd files must
be included.

Fixes: fbb5c060 ("kbuild: add syscall table generation to scripts/Makefile.asm-headers")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Closes: https://lore.kernel.org/lkml/CAHk-=wibB7SvXnUftBgAt+4-3vEKRpvEgBeDEH=i=j2GvDitoA@mail.gmail.com/Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/Makefile.asm-headers

@@ -87,12 +87,20 @@ $(obj)/unistd_compat_%.h: $(syscalltbl) $(syshdr) FORCE
 $(obj)/syscall_table_%.h: $(syscalltbl) $(systbl) FORCE
 	$(call if_changed,systbl)
 
+targets := $(syscall-y)
+
 # Create output directory. Skip it if at least one old header exists
 # since we know the output directory already exists.
 ifeq ($(old-headers),)
 $(shell mkdir -p $(obj))
 endif
 
+PHONY += FORCE
+
 FORCE:
 
+existing-targets := $(wildcard $(sort $(targets)))
+
+-include $(foreach f,$(existing-targets),$(dir $(f)).$(notdir $(f)).cmd)
+
 .PHONY: $(PHONY)