make sure the standard library compiles properly with synthetic targets

It might happen that a synthetic target name does not match one of the
hardcoded ones in std's build script, causing std to fail to build. This
commit changes the std build script avoid including the restricted-std
feature unconditionally when a synthetic target is being built.

Author: pietroalbini

std/build.rs

@@ -36,6 +36,8 @@ fn main() {
         || target.contains("nintendo-3ds")
         || target.contains("vita")
         || target.contains("nto")
+        // See src/bootstrap/synthetic_targets.rs
+        || env::var("RUSTC_BOOTSTRAP_SYNTHETIC_TARGET").is_ok()
     {
         // These platforms don't have any special requirements.
     } else {