Fix undefined symbol "custom panic" in BPF test programs (#13840)

* Implements missing "custom_panic" symbol

programs/bpf/rust/deprecated_loader/src/lib.rs

@@ -20,6 +20,12 @@ fn return_sstruct() -> SStruct {
     SStruct { x: 1, y: 2, z: 3 }
 }
 
+#[no_mangle]
+fn custom_panic(info: &core::panic::PanicInfo<'_>) {
+    // Full panic reporting
+    msg!(&format!("{}", info));
+}
+
 entrypoint_deprecated!(process_instruction);
 fn process_instruction(
     program_id: &Pubkey,