Mark repository as configurable (#515)

This allows re-using extracted repository contents
(bazelbuild/bazel#12227, see release notes of [Bazel
8.3](https://github.com/bazelbuild/bazel/releases/tag/8.3.0)).

Author: mering

toolchain/internal/configure.bzl

@@ -69,7 +69,7 @@ def llvm_config_impl(rctx):
     os = _os(rctx)
     if os == "windows":
         _empty_repository(rctx)
-        return
+        return None
     arch = _arch(rctx)
 
     if not rctx.attr.toolchain_roots:
@@ -83,7 +83,7 @@ def llvm_config_impl(rctx):
     if not llvm_version:
         # LLVM version missing for (os, arch)
         _empty_repository(rctx)
-        return
+        return None
     use_absolute_paths_llvm = rctx.attr.absolute_paths
     use_absolute_paths_sysroot = use_absolute_paths_llvm
 
@@ -227,6 +227,11 @@ def llvm_config_impl(rctx):
         },
     )
 
+    if hasattr(rctx, "repo_metadata"):
+        return rctx.repo_metadata(reproducible = True)
+    else:
+        return None
+
 def _cc_toolchains_str(
         rctx,
         workspace_name,