use acquire ordering for reading the futex value on macOS

Author: joboet

src/shims/unix/macos/sync.rs

@@ -140,7 +140,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         let layout = this.machine.layouts.uint(Size::from_bytes(size)).unwrap();
         let futex_val = this
-            .read_scalar_atomic(&this.ptr_to_mplace(ptr, layout), AtomicReadOrd::Relaxed)?
+            .read_scalar_atomic(&this.ptr_to_mplace(ptr, layout), AtomicReadOrd::Acquire)?
             .to_bits(Size::from_bytes(size))?;
 
         let Some(futex) = this.get_sync_or_init(ptr, |_| {