add lib.miri.rs file for miri-test-libstd

Author: RalfJung

src/lib.miri.rs

@@ -0,0 +1,5 @@
+//! Grep bootstrap for `MIRI_REPLACE_LIBRS_IF_NOT_TEST` to learn what this is about.
+#![no_std]
+#![feature(rustc_private)]
+extern crate compiler_builtins as real;
+pub use real::*;