Add a basic Core.Print function for ints. (#4078)

We'd been discussing that explorer remains necessary for print, and I
was wondering if this kind of approach would be okay (we _probably_ want
this to work, based on #2110, albeit with more overloads -- but I don't
think there's a good way to support overloads at the moment).

```
╚╡../bazel-bin/examples/sieve
2
3
5
7
11
13
17
19
23
29
31
37
41
43
...
```

toolchain/codegen/codegen.cpp

@@ -29,10 +29,9 @@ auto CodeGen::Make(llvm::Module& module, llvm::StringRef target_triple,
   constexpr llvm::StringLiteral Features = "";
 
   llvm::TargetOptions target_opts;
-  std::optional<llvm::Reloc::Model> reloc_model;
   CodeGen codegen(module, errors);
   codegen.target_machine_.reset(target->createTargetMachine(
-      target_triple, CPU, Features, target_opts, reloc_model));
+      target_triple, CPU, Features, target_opts, llvm::Reloc::PIC_));
   return codegen;
 }