Allow unused unsafe for vxworks target only

Author: B I Mohammed Abbas

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

@@ -120,7 +120,7 @@ impl FileDesc {
         (&mut me).read_to_end(buf)
     }
 
-    #[allow(unused_unsafe)]
+    #[cfg_attr(target_os = "vxworks", allow(unused_unsafe))]
     pub fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
         #[cfg(not(any(
             all(target_os = "linux", not(target_env = "musl")),
@@ -314,7 +314,7 @@ impl FileDesc {
         cfg!(not(any(target_os = "espidf", target_os = "horizon", target_os = "vita")))
     }
 
-    #[allow(unused_unsafe)]
+    #[cfg_attr(target_os = "vxworks", allow(unused_unsafe))]
     pub fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
         #[cfg(not(any(
             all(target_os = "linux", not(target_env = "musl")),