Reduce dependence on the target name

The target name can be anything with custom target specs. Matching on
fields inside the target spec is much more robust than matching on the
target name.

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -47,6 +47,7 @@ pub(crate) fn get_linker<'a>(
     self_contained: bool,
     target_cpu: &'a str,
 ) -> Box<dyn Linker + 'a> {
+    // FIXME(cc-rs#1265) pass only target arch to find_tool()
     let msvc_tool = windows_registry::find_tool(sess.opts.target_triple.tuple(), "link.exe");
 
     // If our linker looks like a batch script on Windows then to execute this