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

* Implements missing "custom_panic" symbol

programs/bpf/rust/sha256/src/lib.rs

@@ -2,6 +2,7 @@
 
 extern crate solana_program;
 use solana_program::{
+    custom_panic_default,
     hash::{hashv, Hasher},
     msg,
 };
@@ -22,6 +23,8 @@ pub extern "C" fn entrypoint(_input: *mut u8) -> u64 {
     0
 }
 
+custom_panic_default!();
+
 #[cfg(test)]
 mod test {
     use super::*;