Fix restricted_std flag for custom platforms

Author: NathanRoyer

library/std/build.rs

@@ -1,6 +1,8 @@
 use std::env;
 
 fn main() {
+    let is_custom_os = || env::var("CARGO_CFG_TARGET_OS").as_deref() == Ok("custom");
+
     println!("cargo:rerun-if-changed=build.rs");
     let target = env::var("TARGET").expect("TARGET was not set");
     if target.contains("freebsd") {
@@ -37,6 +39,7 @@ fn main() {
         || target.contains("nintendo-3ds")
         || target.contains("vita")
         || target.contains("nto")
+        || is_custom_os()
         // See src/bootstrap/synthetic_targets.rs
         || env::var("RUSTC_BOOTSTRAP_SYNTHETIC_TARGET").is_ok()
     {

library/std/src/env.rs

@@ -926,6 +926,7 @@ pub mod consts {
     /// - dragonfly
     /// - netbsd
     /// - openbsd
+    /// - custom
     /// - solaris
     /// - android
     /// - windows

library/std/src/sys_common/mod.rs

@@ -44,6 +44,7 @@ cfg_if::cfg_if! {
 
 cfg_if::cfg_if! {
     if #[cfg(any(target_os = "l4re",
+                 target_os = "custom",
                  feature = "restricted-std",
                  all(target_family = "wasm", not(target_os = "emscripten")),
                  all(target_vendor = "fortanix", target_env = "sgx")))] {