Auto merge of rust-lang#30662 - simartin:issue_30592, r=alexcrichton

Fixes rust-lang#30592, a fallout of rust-lang@cd1848a

Author: bors

src/liballoc/lib.rs

@@ -95,8 +95,15 @@
 
 #![feature(needs_allocator)]
 
+// Issue# 30592: Systematically use alloc_system during stage0 since jemalloc
+// might be unavailable or disabled
+#![cfg_attr(stage0, feature(alloc_system))]
+
 #![cfg_attr(test, feature(test, rustc_private, box_heap))]
 
+#[cfg(stage0)]
+extern crate alloc_system;
+
 // Allow testing this library
 
 #[cfg(test)]