ASP-based solver: fix reusing externals on linux (#44316)

We need to tell clingo the libc compatibility of external nodes
in buildcaches or stores, to allow reuse.

var/spack/repos/builtin.mock/packages/sombrero/package.py

@@ -0,0 +1,16 @@
+# Copyright 2013-2024 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack.package import *
+
+
+class Sombrero(Package):
+    """Simple package with a dependency on an external spec."""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/b-1.0.tar.gz"
+
+    version("1.0", md5="0123456789abcdef0123456789abcdef")
+    depends_on("externaltool")