Kbuild: lto: fix module versionings mismatch in GNU make 3.X

When building modules(CONFIG_...=m), I found some of module versions
are incorrect and set to 0.
This can be found in build log for first clean build which shows

WARNING: EXPORT symbol "XXXX" [drivers/XXX/XXX.ko] version generation failed,
symbol will not be versioned.

But in second build(incremental build), the WARNING disappeared and the
module version becomes valid CRC and make someone who want to change
modules without updating kernel image can't insert their modules.

The problematic code is
+	$(foreach n, $(filter-out FORCE,$^),				\
+		$(if $(wildcard $(n).symversions),			\
+			; cat $(n).symversions >> $@.symversions))

For example:
  rm -f fs/notify/built-in.a.symversions    ; rm -f fs/notify/built-in.a; \
llvm-ar cDPrST fs/notify/built-in.a fs/notify/fsnotify.o \
fs/notify/notification.o fs/notify/group.o ...

`foreach n` shows nothing to `cat` into $(n).symversions because
`if $(wildcard $(n).symversions)` return nothing, but actually
they do exist during this line was executed.

-rw-r--r-- 1 root root 168580 Jun 13 19:10 fs/notify/fsnotify.o
-rw-r--r-- 1 root root    111 Jun 13 19:10 fs/notify/fsnotify.o.symversions

The reason is the $(n).symversions are generated at runtime, but
Makefile wildcard function expends and checks the file exist or not
during parsing the Makefile.

Thus fix this by use `test` shell command to check the file
existence in runtime.

Rebase from both:
1. [https://lore.kernel.org/lkml/20210616080252.32046-1-lecopzer.chen@mediatek.com/]
2. [https://lore.kernel.org/lkml/20210702032943.7865-1-lecopzer.chen@mediatek.com/]

Fixes: 38e89184

 ("kbuild: lto: fix module versioning")
Co-developed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Lecopzer Chen <lecopzer.chen@mediatek.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>