Add support for deprecated loader (#11946)

sdk/src/bpf_loader_deprecated.rs

@@ -1 +1,14 @@
+//! @brief The original and now deprecated Solana BPF loader.
+//!
+//! The BPF loader is responsible for loading, finalizing, and executing BPF
+//! programs.
+//!
+//! This loader is deprecated, and it is strongly encouraged to build for and
+//! deploy to the latest BPF loader.  For more information see `bpf_loader.rs`
+//!
+//! The program format may change between loaders, and it is crucial to build
+//! your program against the proper entrypoint semantics.  All programs being
+//! deployed to this BPF loader must build against the deprecated entrypoint
+//! version located in `entrypoint_deprecated.rs`.
+
 crate::declare_id!("BPFLoader1111111111111111111111111111111111");