Fix warnings when checking armv6k-nintendo-3ds

Also fix one instance of unsafe_op_in_unsafe_fn that's specific to
horizon + vita - most others should be common with other code.

Author: ian-h-chamberlain

library/std/src/os/horizon/raw.rs

@@ -9,6 +9,7 @@
             definitions"
 )]
 #![allow(deprecated)]
+#![allow(dead_code)]
 
 use crate::os::raw::c_long;
 use crate::os::unix::raw::{gid_t, uid_t};

library/std/src/sys/pal/unix/alloc.rs

@@ -67,7 +67,7 @@ cfg_if::cfg_if! {
     ))] {
         #[inline]
         unsafe fn aligned_malloc(layout: &Layout) -> *mut u8 {
-            libc::memalign(layout.align(), layout.size()) as *mut u8
+            unsafe { libc::memalign(layout.align(), layout.size()) as *mut u8 }
         }
     } else {
         #[inline]

library/std/src/sys/pal/unix/process/process_unsupported.rs

@@ -1,4 +1,3 @@
-use crate::fmt;
 use crate::io;
 use crate::num::NonZero;
 use crate::sys::pal::unix::unsupported::*;