gcc-plugins: test plugin support in Kconfig and clean up Makefile

Run scripts/gcc-plugin.sh from Kconfig so that users can enable
GCC_PLUGINS only when the compiler supports building plugins.

Kconfig defines a new symbol, PLUGIN_HOSTCC.  This will contain
the compiler (g++ or gcc) used for building plugins, or empty
if the plugin can not be supported at all.

This allows us to remove all ugly testing in Makefile.gcc-plugins.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Kees Cook <keescook@chromium.org>