gcc-plugins: Explicitly document purpose and deprecation schedule

GCC plugins should only exist when some compiler feature needs to be
proven but does not exist in either GCC nor Clang. For example, if a
desired feature is already in Clang, it should be added to GCC upstream.
Document this explicitly.

Additionally, mark the plugins with matching upstream GCC features as
removable past their respective GCC versions.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: linux-hardening@vger.kernel.org
Cc: linux-kbuild@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Cc: llvm@lists.linux.dev
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20211020173554.38122-2-keescook@chromium.org

scripts/gcc-plugins/Kconfig

@@ -37,6 +37,8 @@ config GCC_PLUGIN_CYC_COMPLEXITY
 
 config GCC_PLUGIN_SANCOV
 	bool
+	# Plugin can be removed once the kernel only supports GCC 6+
+	depends on !CC_HAS_SANCOV_TRACE_PC
 	help
 	  This plugin inserts a __sanitizer_cov_trace_pc() call at the start of
 	  basic blocks. It supports all gcc versions with plugin support (from
@@ -83,8 +85,6 @@ config GCC_PLUGIN_RANDSTRUCT
 	  the existing seed and will be removed by a make mrproper or
 	  make distclean.
 
-	  Note that the implementation requires gcc 4.7 or newer.
-
 	  This plugin was ported from grsecurity/PaX. More information at:
 	   * https://grsecurity.net/
 	   * https://pax.grsecurity.net/