Pass tune-cpu to LLVM

I think this is how it should work...

compiler/rustc_session/src/options.rs

@@ -1079,6 +1079,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "show extended diagnostic help (default: no)"),
     terminal_width: Option<usize> = (None, parse_opt_uint, [UNTRACKED],
         "set the current terminal width"),
+    tune_cpu: Option<String> = (None, parse_opt_string, [TRACKED],
+        "select processor to schedule for (`rustc --print target-cpus` for details)"),
     thinlto: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "enable ThinLTO when possible"),
     // We default to 1 here since we want to behave like