Split out Rust BPF no-std stuff (#4968)

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

@@ -4,6 +4,8 @@
 #![allow(unreachable_code)]
 #![allow(unused_attributes)]
 
+#[cfg(not(test))]
+extern crate solana_sdk_bpf_no_std;
 extern crate solana_sdk_bpf_utils;
 
 use solana_sdk_bpf_utils::entrypoint::*;