gcc-plugins: Automate make rule generation

There's no reason to repeat the same names in the Makefile when the .so
files have already been listed. The .o list can be generated from them.

Reported-by: PaX Team <pageexec@freemail.hu>
Signed-off-by: Emese Revfy <re.emese@gmail.com>
[kees: clarified commit message]
Signed-off-by: Kees Cook <keescook@chromium.org>