Ignore unnecessary unsafe warnings

This is a work-around for a libc issue:
rust-lang/libc#1888.

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

@@ -6,7 +6,6 @@ mod tests;
 use crate::cmp;
 use crate::io::{self, Initializer, IoSlice, IoSliceMut, Read};
 use crate::mem;
-#[cfg(not(any(target_os = "redox", target_env = "newlib")))]
 use crate::sys::cvt;
 use crate::sys_common::AsInner;
 

library/std/src/sys/unix/process/process_unix.rs

@@ -459,18 +459,38 @@ impl ExitStatus {
     }
 
     fn exited(&self) -> bool {
-        unsafe { libc::WIFEXITED(self.0) }
+        // On Linux-like OSes this function is safe, on others it is not. See
+        // libc issue: https://github.com/rust-lang/libc/issues/1888.
+        #[cfg_attr(
+            any(target_os = "linux", target_os = "android", target_os = "emscripten"),
+            allow(unused_unsafe)
+        )]
+        unsafe {
+            libc::WIFEXITED(self.0)
+        }
     }
 
     pub fn success(&self) -> bool {
         self.code() == Some(0)
     }
 
     pub fn code(&self) -> Option<i32> {
+        // On Linux-like OSes this function is safe, on others it is not. See
+        // libc issue: https://github.com/rust-lang/libc/issues/1888.
+        #[cfg_attr(
+            any(target_os = "linux", target_os = "android", target_os = "emscripten"),
+            allow(unused_unsafe)
+        )]
         if self.exited() { Some(unsafe { libc::WEXITSTATUS(self.0) }) } else { None }
     }
 
     pub fn signal(&self) -> Option<i32> {
+        // On Linux-like OSes this function is safe, on others it is not. See
+        // libc issue: https://github.com/rust-lang/libc/issues/1888.
+        #[cfg_attr(
+            any(target_os = "linux", target_os = "android", target_os = "emscripten"),
+            allow(unused_unsafe)
+        )]
         if !self.exited() { Some(unsafe { libc::WTERMSIG(self.0) }) } else { None }
     }
 }