gcc-plugins: remove unused GCC_PLUGIN_SUBDIR

GCC_PLUGIN_SUBDIR has never been used.  If you really need this in
the future, please re-add it then.

For now, the code is unused. Remove.

'export HOSTLIBS' is not necessary either.

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