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.

std/src/os/horizon/mod.rs

@@ -1,5 +1,6 @@
 //! Definitions for Horizon OS
 
+#![forbid(unsafe_op_in_unsafe_fn)]
 #![stable(feature = "raw_ext", since = "1.1.0")]
 
 pub mod fs;

std/src/os/horizon/raw.rs

@@ -38,6 +38,7 @@ pub type time_t = libc::time_t;
 #[repr(C)]
 #[derive(Clone)]
 #[stable(feature = "raw_ext", since = "1.1.0")]
+#[allow(dead_code)] // This exists for parity with other `raw` modules, but isn't actually used.
 pub struct stat {
     #[stable(feature = "raw_ext", since = "1.1.0")]
     pub st_dev: dev_t,

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]

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::*;