Expose a feature to force use of alloc_system, teach rustbuild

This fixes jemalloc-less local rebuilds, where we tell cargo that
we're actually stage1

src/rustc/std_shim/Cargo.toml

@@ -38,4 +38,5 @@ core = { path = "../../libcore" }
 backtrace = ["std/backtrace"]
 debug-jemalloc = ["std/debug-jemalloc"]
 jemalloc = ["std/jemalloc"]
+force_alloc_system = ["std/force_alloc_system"]
 panic-unwind = ["std/panic-unwind"]