Update std doctests for android

This updates some doctests that fail to run on android. We will soon be
supporting cross-compiled doctests, and the `arm-android` job fails to
run these tests.

In summary:
- Android re-exports some traits from linux under a different path.
- Android doesn't seem to have common unix utilities like `true`,
  `false`, or `whoami`, so these are disabled.

Author: ehuss

std/src/os/net/linux_ext/addr.rs

@@ -23,7 +23,10 @@ pub trait SocketAddrExt: Sealed {
     ///
     /// ```no_run
     /// use std::os::unix::net::{UnixListener, SocketAddr};
+    /// #[cfg(target_os = "linux")]
     /// use std::os::linux::net::SocketAddrExt;
+    /// #[cfg(target_os = "android")]
+    /// use std::os::android::net::SocketAddrExt;
     ///
     /// fn main() -> std::io::Result<()> {
     ///     let addr = SocketAddr::from_abstract_name(b"hidden")?;
@@ -48,7 +51,10 @@ pub trait SocketAddrExt: Sealed {
     ///
     /// ```no_run
     /// use std::os::unix::net::{UnixListener, SocketAddr};
+    /// #[cfg(target_os = "linux")]
     /// use std::os::linux::net::SocketAddrExt;
+    /// #[cfg(target_os = "android")]
+    /// use std::os::android::net::SocketAddrExt;
     ///
     /// fn main() -> std::io::Result<()> {
     ///     let name = b"hidden";

std/src/os/net/linux_ext/socket.rs

@@ -27,7 +27,10 @@ pub trait UnixSocketExt: Sealed {
     ///
     /// ```no_run
     /// #![feature(unix_socket_ancillary_data)]
+    /// #[cfg(target_os = "linux")]
     /// use std::os::linux::net::UnixSocketExt;
+    /// #[cfg(target_os = "android")]
+    /// use std::os::android::net::UnixSocketExt;
     /// use std::os::unix::net::UnixDatagram;
     ///
     /// fn main() -> std::io::Result<()> {

std/src/os/net/linux_ext/tcp.rs

@@ -25,7 +25,10 @@ pub trait TcpStreamExt: Sealed {
     /// ```no_run
     /// #![feature(tcp_quickack)]
     /// use std::net::TcpStream;
+    /// #[cfg(target_os = "linux")]
     /// use std::os::linux::net::TcpStreamExt;
+    /// #[cfg(target_os = "android")]
+    /// use std::os::android::net::TcpStreamExt;
     ///
     /// let stream = TcpStream::connect("127.0.0.1:8080")
     ///         .expect("Couldn't connect to the server...");
@@ -43,7 +46,10 @@ pub trait TcpStreamExt: Sealed {
     /// ```no_run
     /// #![feature(tcp_quickack)]
     /// use std::net::TcpStream;
+    /// #[cfg(target_os = "linux")]
     /// use std::os::linux::net::TcpStreamExt;
+    /// #[cfg(target_os = "android")]
+    /// use std::os::android::net::TcpStreamExt;
     ///
     /// let stream = TcpStream::connect("127.0.0.1:8080")
     ///         .expect("Couldn't connect to the server...");

std/src/process.rs

@@ -1348,7 +1348,7 @@ impl Output {
     ///
     /// ```
     /// #![feature(exit_status_error)]
-    /// # #[cfg(unix)] {
+    /// # #[cfg(all(unix, not(target_os = "android")))] {
     /// use std::process::Command;
     /// assert!(Command::new("false").output().unwrap().exit_ok().is_err());
     /// # }
@@ -1695,7 +1695,7 @@ impl From<io::Stdout> for Stdio {
     /// # Ok(())
     /// # }
     /// #
-    /// # if cfg!(unix) {
+    /// # if cfg!(all(unix, not(target_os = "android"))) {
     /// #     test().unwrap();
     /// # }
     /// ```
@@ -1724,7 +1724,7 @@ impl From<io::Stderr> for Stdio {
     /// # Ok(())
     /// # }
     /// #
-    /// # if cfg!(unix) {
+    /// # if cfg!(all(unix, not(target_os = "android"))) {
     /// #     test().unwrap();
     /// # }
     /// ```
@@ -1907,7 +1907,7 @@ impl crate::sealed::Sealed for ExitStatusError {}
 ///
 /// ```
 /// #![feature(exit_status_error)]
-/// # if cfg!(unix) {
+/// # if cfg!(all(unix, not(target_os = "android"))) {
 /// use std::process::{Command, ExitStatusError};
 ///
 /// fn run(cmd: &str) -> Result<(), ExitStatusError> {
@@ -1950,7 +1950,7 @@ impl ExitStatusError {
     ///
     /// ```
     /// #![feature(exit_status_error)]
-    /// # #[cfg(unix)] {
+    /// # #[cfg(all(unix, not(target_os = "android")))] {
     /// use std::process::Command;
     ///
     /// let bad = Command::new("false").status().unwrap().exit_ok().unwrap_err();
@@ -1975,7 +1975,7 @@ impl ExitStatusError {
     /// ```
     /// #![feature(exit_status_error)]
     ///
-    /// # if cfg!(unix) {
+    /// # if cfg!(all(unix, not(target_os = "android"))) {
     /// use std::num::NonZero;
     /// use std::process::Command;
     ///