an attempt to fix the repl tests

crates/compiler/gen_llvm/src/llvm/externs.rs

@@ -155,10 +155,32 @@ pub fn add_default_roc_externs(env: &Env<'_, '_, '_>) {
             }
         }
 
+        unreachable_function(env, "roc_getppid");
+        unreachable_function(env, "roc_mmap");
+        unreachable_function(env, "roc_send_signal");
+        unreachable_function(env, "roc_shm_open");
+
         add_sjlj_roc_panic(env)
     }
 }
 
+fn unreachable_function(env: &Env, name: &str) {
+    // The type of this function (but not the implementation) should have
+    // already been defined by the builtins, which rely on it.
+    let fn_val = env.module.get_function(name).unwrap();
+
+    // Add a basic block for the entry point
+    let entry = env.context.append_basic_block(fn_val, "entry");
+
+    env.builder.position_at_end(entry);
+
+    env.builder.build_unreachable();
+
+    if cfg!(debug_assertions) {
+        crate::llvm::build::verify_fn(fn_val);
+    }
+}
+
 pub fn add_sjlj_roc_panic(env: &Env<'_, '_, '_>) {
     let ctx = env.context;
     let module = env.module;