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: dee2b702bcf0 ("kconfig: Add support for -Wimplicit-fallthrough")
Reported-by: Nathan Chancellor <[e-mail]>
Co-developed-by: Nathan Chancellor <[e-mail]>
Signed-off-by: Nathan Chancellor <[e-mail]>
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

init/Kconfig

···
 
 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)
 
 #