Make clingo the default solver (#25502)

Modifications:
- [x] Change `defaults/config.yaml`
- [x] Add a fix for bootstrapping patchelf from sources if `compilers.yaml` is empty
- [x] Make `SPACK_TEST_SOLVER=clingo` the default for unit-tests
- [x] Fix package failures in the e4s pipeline

Caveats:
1. CentOS 6 still uses the original concretizer as it can't connect to the buildcache due to issues with `ssl` (bootstrapping from sources requires a C++14 capable compiler)
1. I had to update the image tag for GitlabCI in e699f14.  
1. libtool v2.4.2 has been deprecated and other packages received some update

lib/spack/spack/bootstrap.py

@@ -489,6 +489,16 @@ def _bootstrap_config_scopes():
 
 @contextlib.contextmanager
 def ensure_bootstrap_configuration():
+    # We may need to compile code from sources, so ensure we have compilers
+    # for the current platform before switching parts.
+    arch = spack.architecture.default_arch()
+    arch = spack.spec.ArchSpec(str(arch))  # The call below expects an ArchSpec object
+    if not spack.compilers.compilers_for_arch(arch):
+        compiler_cmd = spack.main.SpackCommand('compiler')
+        compiler_cmd(
+            'find', output=os.devnull, error=os.devnull, fail_on_error=False
+        )
+
     bootstrap_store_path = store_path()
     with spack.environment.deactivate_environment():
         with spack.architecture.use_platform(spack.architecture.real_platform()):

lib/spack/spack/test/conftest.py

@@ -585,7 +585,7 @@ def configuration_dir(tmpdir_factory, linux_os):
     tmpdir.ensure('user', dir=True)
 
     # Slightly modify config.yaml and compilers.yaml
-    solver = os.environ.get('SPACK_TEST_SOLVER', 'original')
+    solver = os.environ.get('SPACK_TEST_SOLVER', 'clingo')
     config_yaml = test_config.join('config.yaml')
     modules_root = tmpdir_factory.mktemp('share')
     tcl_root = modules_root.ensure('modules', dir=True)