kbuild: Fix -Wimplicit-fallthrough=5 error for GCC 5.x and 6.x

Bugzilla: https://bugzilla.redhat.com/2159468

commit 158ea2d
Author: Gustavo A. R. Silva <gustavoars@kernel.org>
Date:   Sun Nov 14 20:48:44 2021 -0600

    kbuild: Fix -Wimplicit-fallthrough=5 error for GCC 5.x and 6.x

    -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: dee2b70 ("kconfig: Add support for -Wimplicit-fallthrough")
    Reported-by: Nathan Chancellor <nathan@kernel.org>
    Co-developed-by: Nathan Chancellor <nathan@kernel.org>
    Signed-off-by: Nathan Chancellor <nathan@kernel.org>
    Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Signed-off-by: Eric Chanudet <echanude@redhat.com>

Author: eric-ch

init/Kconfig

@@ -879,7 +879,7 @@ 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)
 
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