Utilize cfg(bootstrap) over cfg(stage0)

Also removes stage1, stage2 cfgs being passed to rustc to ensure that
stage1 and stage2 are only differentiated as a group (i.e., only through
not bootstrap).

src/libcore/num/mod.rs

@@ -50,7 +50,7 @@ assert_eq!(size_of::<Option<core::num::", stringify!($Ty), ">>(), size_of::<", s
                 #[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
                 #[repr(transparent)]
                 #[rustc_layout_scalar_valid_range_start(1)]
-                #[cfg_attr(not(stage0), rustc_nonnull_optimization_guaranteed)]
+                #[cfg_attr(not(bootstrap), rustc_nonnull_optimization_guaranteed)]
                 pub struct $Ty($Int);
             }