Pass -Z no-codegen when invoking rustc.

We only need crate metadata and MIR, not the built object files, so this
allows us to skip unecessary work when building `libstd`. It also
prevents us from trying to codegen the `try` panicking intrinsic, which
assumes that `libpanic_unwind` is being compiled for the target
platform. We built `libpanic_unwind` with `cfg(miri)`, which may
invalidate these assumptions (e.g. the `eh_catch_typeinfo` lang item is
no longer defined).

README.md

@@ -172,6 +172,11 @@ Several `-Z` flags are relevant for Miri:
   sets this flag per default.
 * `-Zmir-emit-retag` controls whether `Retag` statements are emitted. Miri
   enables this per default because it is needed for validation.
+* `-Zno-codegen` makes rustc skip generating LLVM IR for MIR (though it still
+  writes out empty object files). This is needed to prevent rustc from trying
+  to codegen certain panic-related intrinsics, which will not work with a
+  `libpanic_unwind` compiled in `cfg(miri)` mode. It also speeds up
+  building our custom libstd.
 
 Moreover, Miri recognizes some environment variables:
 

src/lib.rs

@@ -51,5 +51,5 @@ pub use crate::eval::{eval_main, create_ecx, MiriConfig};
 /// Insert rustc arguments at the beginning of the argument list that Miri wants to be
 /// set per default, for maximal validation power.
 pub fn miri_default_args() -> &'static [&'static str] {
-    &["-Zalways-encode-mir", "-Zmir-emit-retag", "-Zmir-opt-level=0", "--cfg=miri"]
+    &["-Zalways-encode-mir", "-Zmir-emit-retag", "-Zmir-opt-level=0", "-Zno-codegen", "--cfg=miri"]
 }