initial support for clang compiler plugins

The plugin is intentionally built using a custom Makefile,
because it's used by gbuild, so I don't want to build the plugin
using gbuild too. It is also intentionally not placed under workdir/,
as that is cleaned by 'make clean', the plugin is cleaned only
by 'make distclean', so that cleaning it doesn't cause ccache misses.
No actual functionality in the plugin itself yet.

Change-Id: Ic05eba8d6260eec123c9e699eb5385abfe1b832f

compilerplugins/README

@@ -0,0 +1,27 @@
+Compiler plugins.
+
+== Overview ==
+
+This directory contains code for compiler plugins. These are used to perform
+additional actions during compilation (such as additional warnings) and
+also to perform mass code refactoring.
+
+Currently only the Clang compiler is supported (http://clang.llvm.org).
+
+== Usage ==
+
+Compiler plugins are enabled automatically by --enable-dbgutil if Clang headers
+are found or explicitly using --enable-compiler-plugins.
+
+
+== Functionality ==
+
+=== Compile plugin ===
+
+The compile plugin is used during normal compilation to perform additional checks.
+All warnings and errors are marked '[loplugin]' in the message.
+
+
+== Code documentation / howtos ==
+
+TBD