gcc-plugins: move GCC version check for PowerPC to Kconfig

For PowerPC, GCC 5.2 is the requirement for GCC plugins.  Move the
version check to Kconfig so that the GCC plugin menus will be hidden
if an older compiler is in use.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>