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

etc/spack/defaults/config.yaml

@@ -160,11 +160,10 @@ config:
   #       sufficiently for many specs.
   #
   #   'clingo': Uses a logic solver under the hood to solve DAGs with full
-  #       backtracking and optimization for user preferences.
+  #       backtracking and optimization for user preferences. Spack will
+  #       try to bootstrap the logic solver, if not already available.
   #
-  # 'clingo' currently requires the clingo ASP solver to be installed and
-  # built with python bindings. 'original' is built in.
-  concretizer: original
+  concretizer: clingo
 
 
   # How long to wait to lock the Spack installation database. This lock is used