fix data race in ReentrantLock fallback for targets without 64bit atomics

Author: RalfJung

tests/many-seeds/reentrant-lock.rs

@@ -0,0 +1,19 @@
+#![feature(reentrant_lock)]
+//! This is a regression test for
+//! <https://rust-lang.zulipchat.com/#narrow/channel/269128-miri/topic/reentrant.20lock.20failure.20on.20musl>.
+
+use std::cell::Cell;
+use std::sync::ReentrantLock;
+use std::thread;
+
+static LOCK: ReentrantLock<Cell<i32>> = ReentrantLock::new(Cell::new(0));
+
+fn main() {
+    for _ in 0..20 {
+        thread::spawn(move || {
+            let val = LOCK.lock();
+            val.set(val.get() + 1);
+            drop(val);
+        });
+    }
+}