libffi: Add HOWTO_MERGE, autogen.sh and merge.sh

Add scripts for syncing with libffi upstream:

1. Clone libffi repo.
2. Checkout the specific commit.
3. Remove the unused files.
4. Add new files and remove old files if needed.

	* HOWTO_MERGE: New file.
	* autogen.sh: Likewise.
	* merge.sh: Likewise.