toolchain: add a program for compiling ipc interface definitions

ifc can be used to compile .if files into self-contained header-only C
libraries, which can be used to send/receive messages that conform
to the described interface.

toolchain/ifc/backend.h

@@ -0,0 +1,13 @@
+#ifndef IFC_BACKEND_H_
+#define IFC_BACKEND_H_
+
+struct interface_definition;
+
+struct backend {
+	const char *b_name;
+	int (*b_emit)(const struct interface_definition *);
+};
+
+extern const struct backend *c_mpc_backend(void);
+
+#endif