Fallback to unknown vendor with vendored target triples

When a target triple is searched for, only exact matches with be
returned. This change will fallback to an "unknown" vendor in the
target triple if one with the specific vendor can not be found.

Author: zachreizner

src/librustc_target/spec/mod.rs

@@ -247,11 +247,19 @@ macro_rules! supported_targets {
                         // run-time that the parser works correctly
                         t = Target::from_json(t.to_json())?;
                         debug!("Got builtin target: {:?}", t);
-                        Ok(t)
+                        return Ok(t)
                     },
                 )+
-                _ => Err(format!("Unable to find target: {}", target))
+                _ => {},
             }
+            let mut triple: Vec<&str> = target.splitn(3, '-').collect();
+            if triple.len() >= 3 {
+                if triple[1] != "unknown" {
+                    triple[1] = "unknown";
+                    return load_specific(&triple.join("-"))
+                }
+            }
+            Err(format!("Unable to find target: {}", target))
         }
 
         pub fn get_targets() -> Box<dyn Iterator<Item=String>> {