[PATCH v3] kbuild: Fix -Wimplicit-fallthrough=5 error for GCC 5.x and 6.x

[Gustavo A. R. Silva]

-Wimplicit-fallthrough=5 was under cc-option because it was only
available in GCC 7.x and newer so the build is now broken for GCC 5.x
and 6.x:

gcc: error: unrecognized command line option '-Wimplicit-fallthrough=5';
did you mean '-Wno-fallthrough'?

Fix this by moving -Wimplicit-fallthrough=5 under cc-option.

Fixes: dee2b702bcf0 ("kconfig: Add support for -Wimplicit-fallthrough")
Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Co-developed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
---
Changes in v3:
 - Keep the original CC_IS_GCC part of the test[1].
   Link: https://lore.kernel.org/linux-hardening/CAHk-=whaJCEwqKbekSNjziY4RMTKzkRkkrOM4MEZQEo0j3HT=Q@xxxxxxxxxxxxxx/ [1]

Changes in v2:
 - Add Fixes tag.

 init/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/init/Kconfig b/init/Kconfig
index 036b750e8d8a..4b7bac10c72d 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -887,7 +887,7 @@ config CC_HAS_INT128
 
 config CC_IMPLICIT_FALLTHROUGH
 	string
-	default "-Wimplicit-fallthrough=5" if CC_IS_GCC
+	default "-Wimplicit-fallthrough=5" if CC_IS_GCC && $(cc-option,-Wimplicit-fallthrough=5)
 	default "-Wimplicit-fallthrough" if CC_IS_CLANG && $(cc-option,-Wunreachable-code-fallthrough)
 
 #
-- 
2.27.0