Rollup merge of rust-lang#66441 - RalfJung:seh, r=oli-obk

libpanic_unwind for Miri: make sure we have the SEH lang items when needed

r? @oli-obk  @alexcrichton This is required to fix the Miri toolstate. Turns out rustc complains when doing codegen for MSVC and these lang items do not exist. For now `cfg(miri)` needs to still be able to codegen (we [plan to change that](rust-lang/miri#1048 (comment)) but that's a larger project requiring improvements to xargo and maybe also cargo; that should not block fixing the toolstate). Yes, this is a hack, but it is inside `cfg(miri)` so I hope this is okay.

Cc @Aaron1011

src/libpanic_unwind/lib.rs

@@ -39,6 +39,11 @@ cfg_if::cfg_if! {
     if #[cfg(miri)] {
         #[path = "miri.rs"]
         mod imp;
+        // On MSVC we need the SEH lang items as well...
+        // This should match the conditions of the `seh.rs` import below.
+        #[cfg(all(target_env = "msvc", not(target_arch = "aarch64")))]
+        #[allow(unused)]
+        mod seh;
     } else if #[cfg(target_os = "emscripten")] {
         #[path = "emcc.rs"]
         mod imp;