Make a disable-jemalloc build work

Fixes #43510

Author: aidanhs

src/libstd/lib.rs

@@ -318,6 +318,16 @@
 
 #![default_lib_allocator]
 
+// Always use alloc_system during stage0 since we don't know if the alloc_*
+// crate the stage0 compiler will pick by default is available (most
+// obviously, if the user has disabled jemalloc in `./configure`).
+// `force_alloc_system` is *only* intended as a workaround for local rebuilds
+// with a rustc without jemalloc.
+#![cfg_attr(any(stage0, feature = "force_alloc_system"), feature(global_allocator))]
+#[cfg(any(stage0, feature = "force_alloc_system"))]
+#[global_allocator]
+static ALLOC: alloc_system::System = alloc_system::System;
+
 // Explicitly import the prelude. The compiler uses this same unstable attribute
 // to import the prelude implicitly when building crates that depend on std.
 #[prelude_import]