kconfig: Add support for -Wimplicit-fallthrough

Add Kconfig support for -Wimplicit-fallthrough for both GCC and Clang.

The compiler option is under configuration CC_IMPLICIT_FALLTHROUGH,
which is enabled by default.

Special thanks to Nathan Chancellor who fixed the Clang bug[1][2]. This
bugfix only appears in Clang 14.0.0, so older versions still contain
the bug and -Wimplicit-fallthrough won't be enabled for them, for now.

This concludes a long journey and now we are finally getting rid
of the unintentional fallthrough bug-class in the kernel, entirely. :)

Link: https://github.com/llvm/llvm-project/commit/9ed4a94d6451046a51ef393cd62f00710820a7e8 [1]
Link: https://bugs.llvm.org/show_bug.cgi?id=51094 [2]
Link: https://github.com/KSPP/linux/issues/115
Link: https://github.com/ClangBuiltLinux/linux/issues/236
Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Co-developed-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Makefile b/Makefile
index d83d72c..9cf9d5c 100644
--- a/Makefile
+++ b/Makefile
@@ -789,7 +789,7 @@
 KBUILD_CFLAGS += $(stackp-flags-y)
 KBUILD_CPPFLAGS += -Qunused-arguments
@@ -801,10 +801,6 @@
 KBUILD_CFLAGS += -mno-global-merge
-# Warn about unmarked fall-throughs in switch statement.
-# Disabled for clang while comment to attribute conversion happens and
-# https://github.com/ClangBuiltLinux/linux/issues/636 is discussed.
-KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough=5,)
 # gcc inanely warns about local variables called 'main'
 KBUILD_CFLAGS += -Wno-main
diff --git a/init/Kconfig b/init/Kconfig
index 45bcaa8..036b750 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -885,6 +885,11 @@
 config CC_HAS_INT128
 	def_bool !$(cc-option,$(m64-flag) -D__SIZEOF_INT128__=0) && 64BIT
+	string
+	default "-Wimplicit-fallthrough=5" if CC_IS_GCC
+	default "-Wimplicit-fallthrough" if CC_IS_CLANG && $(cc-option,-Wunreachable-code-fallthrough)
 # For architectures that know their GCC __int128 support is sound