sancov: Split plugin build from plugin CFLAGS

When the sancov_plugin is enabled, it gets added to gcc-plugin-y which
is used to populate both GCC_PLUGIN (for building the plugin) and
GCC_PLUGINS_CFLAGS (for enabling and options). Instead of adding sancov
to both and then removing it from GCC_PLUGINS_CFLAGS, create a separate
list, gcc-plugin-external-y, which is only added to GCC_PLUGIN.

This will also be used by the coming randstruct build changes.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild@vger.kernel.org
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220503205503.3054173-3-keescook@chromium.org