initial port of the exchange allocator to jemalloc

In stage0, all allocations are 8-byte aligned. Passing a size and
alignment to free is not yet implemented everywhere (0 size and 8 align
are used as placeholders). Fixing this is part of #13994.

Closes #13616

src/libstd/lib.rs

@@ -110,6 +110,7 @@
 // Don't link to std. We are std.
 #![no_std]
 
+#![allow(deprecated)]
 #![deny(missing_doc)]
 
 // When testing libstd, bring in libuv as the I/O backend so tests can print