rustc: Prepare to enable ThinLTO by default

This commit prepares to enable ThinLTO and multiple codegen units in release
mode by default. We've still got a debuginfo bug or two to sort out before
actually turning it on by default.

src/test/run-fail/mir_trans_no_landing_pads.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags: -Z no-landing-pads
+// compile-flags: -Z no-landing-pads -C codegen-units=1
 // error-pattern:converging_fn called
 use std::io::{self, Write};
 

src/test/run-fail/mir_trans_no_landing_pads_diverging.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags: -Z no-landing-pads
+// compile-flags: -Z no-landing-pads -C codegen-units=1
 // error-pattern:diverging_fn called
 use std::io::{self, Write};
 

src/test/run-pass/no-landing-pads.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags: -Z no-landing-pads
+// compile-flags: -Z no-landing-pads -C codegen-units=1
 // ignore-emscripten no threads support
 
 use std::thread;