[perf experiment] (crudely) implement MIR-only rlibs

Author: saethlin

src/bootstrap/src/core/builder.rs

@@ -1371,6 +1371,9 @@ impl<'a> Builder<'a> {
                 rustflags.arg("--cfg=bootstrap");
             }
         }
+        if stage != 0 && mode == Mode::Std {
+            rustflags.arg("-Zcross-crate-inline-threshold=yes");
+        }
 
         let use_new_symbol_mangling = match self.config.rust_new_symbol_mangling {
             Some(setting) => {