fix built-in target detection

previously the logic was accepting wrong triples (like
`x86_64_unknown-linux-musl`) as valid ones (like `x86_64-unknown-linux-musl`) if
they contained an underscore instead of a dash.

fixes rust-lang#33329

Author: Jorge Aparicio

src/librustc_back/target/mod.rs

@@ -71,10 +71,9 @@ macro_rules! supported_targets {
 
         // this would use a match if stringify! were allowed in pattern position
         fn load_specific(target: &str) -> Option<Target> {
-            let target = target.replace("-", "_");
             if false { }
             $(
-                else if target == stringify!($module) {
+                else if target == $triple {
                     let mut t = $module::target();
                     t.options.is_builtin = true;
                     debug!("Got builtin target: {:?}", t);

src/test/run-make/issue-33329/Makefile

@@ -0,0 +1,3 @@
+all:
+	$(RUSTC) --target x86_64_unknown-linux-musl main.rs 2>&1 | \
+		grep "error: Error loading target specification: Could not find specification for target"

src/test/run-make/issue-33329/main.rs

@@ -0,0 +1,11 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {}