fix error codes sign to be negative

src/platform/synchronization.rs

@@ -25,11 +25,12 @@ pub struct z_mutex_t(usize);
 impl_guarded_transmute!(z_mutex_t, ZMutexPtr);
 impl_guarded_transmute!(ZMutexPtr, z_mutex_t);
 
-// try to use the same error codes as in GNU pthreads
-const EBUSY: i8 = 16;
-const EINVAL: i8 = 22;
-const EAGAIN: i8 = 11;
-const EPOISON: i8 = 22; // same as EINVAL
+// using the same error codes as in GNU pthreads, but with negative sign
+// due to convention to return negative values on error
+const EBUSY: i8 = -16;
+const EINVAL: i8 = -22;
+const EAGAIN: i8 = -11;
+const EPOISON: i8 = -22; // same as EINVAL
 
 #[no_mangle]
 #[allow(clippy::missing_safety_doc)]