add write-bytes test

tests/run-pass/write-bytes.rs

@@ -0,0 +1,17 @@
+fn main() {
+    const LENGTH: usize = 10;
+    let mut v: [u64; LENGTH] = [0; LENGTH];
+
+    for idx in 0..LENGTH {
+        assert_eq!(v[idx], 0);
+    }
+
+    unsafe {
+        let p = v.as_mut_ptr();
+        ::std::ptr::write_bytes(p, 0xab, LENGTH);
+    }
+
+    for idx in 0..LENGTH {
+        assert_eq!(v[idx], 0xabababababababab);
+    }
+}