kbuild: mark prepare0 as PHONY to fix external module build

Commit c3ff2a51 ("powerpc/32: add stack protector support")
caused kernel panic on PowerPC when an external module is used with
CONFIG_STACKPROTECTOR because the 'prepare' target was not executed
for the external module build.

Commit e07db28e ("kbuild: fix single target build for external
module") turned it into a build error because the 'prepare' target is
now executed but the 'prepare0' target is missing for the external
module build.

External module on arm/arm64 with CONFIG_STACKPROTECTOR_PER_TASK is
also broken in the same way.

Move 'PHONY += prepare0' to the common place. GNU Make is fine with
missing rule for phony targets. I also removed the comment which is
wrong irrespective of this commit.

I minimize the change so it can be easily backported to 4.20.x

To fix v4.20, please backport e07db28e ("kbuild: fix single target
build for external module"), and then this commit.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=201891
Fixes: e07db28e ("kbuild: fix single target build for external module")
Fixes: c3ff2a51 ("powerpc/32: add stack protector support")
Fixes: 189af465 ("ARM: smp: add support for per-task stack canaries")
Fixes: 0a1213fa ("arm64: enable per-task stack canaries")
Cc: linux-stable <stable@vger.kernel.org> # v4.20
Reported-by: NSamuel Holland <samuel@sholland.org>
Reported-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: NAlexey Kardashevskiy <aik@ozlabs.ru>

Makefile

@@ -955,6 +955,7 @@ ifdef CONFIG_STACK_VALIDATION
   endif
 endif
 
+PHONY += prepare0
 
 ifeq ($(KBUILD_EXTMOD),)
 core-y		+= kernel/ certs/ mm/ fs/ ipc/ security/ crypto/ block/
@@ -1061,8 +1062,7 @@ scripts: scripts_basic scripts_dtc
 # archprepare is used in arch Makefiles and when processed asm symlink,
 # version.h and scripts_basic is processed / created.
 
-# Listed in dependency order
-PHONY += prepare archprepare prepare0 prepare1 prepare2 prepare3
+PHONY += prepare archprepare prepare1 prepare2 prepare3
 
 # prepare3 is used to check if we are building in a separate output directory,
 # and if so do: