Support building solana-program on 32-bit architectures that do not (#21577)

have 64-bit atomics by using a Mutex<u64> on 32-bit architectures.

Currently the usage of atomics are only in functions that support
tests and benchmarks.

sdk/program/src/atomic_u64.rs

@@ -0,0 +1,38 @@
+pub(crate) use implementation::AtomicU64;
+
+#[cfg(target_pointer_width = "64")]
+mod implementation {
+    use std::sync::atomic;
+
+    pub(crate) struct AtomicU64(atomic::AtomicU64);
+
+    impl AtomicU64 {
+        pub(crate) const fn new(initial: u64) -> Self {
+            Self(atomic::AtomicU64::new(initial))
+        }
+
+        pub(crate) fn fetch_add(&self, v: u64) -> u64 {
+            self.0.fetch_add(v, atomic::Ordering::Relaxed)
+        }
+    }
+}
+
+#[cfg(not(target_pointer_width = "64"))]
+mod implementation {
+    use parking_lot::{const_mutex, Mutex};
+
+    pub(crate) struct AtomicU64(Mutex<u64>);
+
+    impl AtomicU64 {
+        pub(crate) const fn new(initial: u64) -> Self {
+            Self(const_mutex(initial))
+        }
+
+        pub(crate) fn fetch_add(&self, v: u64) -> u64 {
+            let mut lock = self.0.lock();
+            let i = *lock;
+            *lock = i + v;
+            i
+        }
+    }
+}