gcc-plugins: Regularize Makefile.gcc-plugins

The layout of Makefile.gcc-plugins had uneven tabs, and the long names
of things made this file a bit hard to quickly visually parse. This
breaks lines and moves options to the same tab depth. While we're at
it, this also adds some comments about the various sections.

Signed-off-by: Kees Cook <keescook@chromium.org>