Fix clingo bootstrapping on rhel + ppc64le

The system Python interpreter on rhel is patched to have
slightly different names for some architectures. This
makes it incompatible with manylinux generated extensions
for ppc64le.

To fix this issue when bootstrapping Spack we generate
on-the-fly symbolic links to the name expected by the
current interpreter if it differs from the default.

Links:
  https://github.com/pypa/manylinux/issues/687
  https://src.fedoraproject.org/fork/churchyard/rpms/python3/blame/00274-fix-arch-names.patch?identifier=test_email-mktime

lib/spack/spack/bootstrap.py

@@ -5,8 +5,11 @@
 from __future__ import print_function
 
 import contextlib
+import fnmatch
 import json
 import os
+import os.path
+import re
 import sys
 
 try:
@@ -81,6 +84,7 @@ def _try_import_from_store(module, abstract_spec_str):
         sys.path.extend(module_paths)
 
         try:
+            _fix_ext_suffix(candidate_spec)
             if _python_import(module):
                 msg = ('[BOOTSTRAP MODULE {0}] The installed spec "{1}/{2}" '
                        'provides the "{0}" Python module').format(
@@ -101,6 +105,55 @@ def _try_import_from_store(module, abstract_spec_str):
     return False
 
 
+def _fix_ext_suffix(candidate_spec):
+    """Fix the external suffixes of Python extensions on the fly for
+    platforms that may need it
+
+    Args:
+        candidate_spec (Spec): installed spec with a Python module
+            to be checked.
+    """
+    # Here we map target families to the patterns expected
+    # by pristine CPython. Only architectures with known issues
+    # are included. Known issues:
+    #
+    # [RHEL + ppc64le]: https://github.com/spack/spack/issues/25734
+    #
+    _suffix_to_be_checked = {
+        'ppc64le': {
+            'glob': '*.cpython-*-powerpc64le-linux-gnu.so',
+            're': r'.cpython-[\w]*-powerpc64le-linux-gnu.so'
+        }
+    }
+
+    # If the current architecture is not problematic return
+    generic_target = archspec.cpu.host().family
+    if str(generic_target) not in _suffix_to_be_checked:
+        return
+
+    # If there's no EXT_SUFFIX (Python < 3.5) or the suffix matches
+    # the expectations, return since the package is surely good
+    ext_suffix = sysconfig.get_config_var('EXT_SUFFIX')
+    if ext_suffix is None:
+        return
+
+    expected = _suffix_to_be_checked[str(generic_target)]
+    if fnmatch.fnmatch(ext_suffix, expected['glob']):
+        return
+
+    # If we are here it means the current interpreter expects different names
+    # than pristine CPython. So:
+    # 1. Find what we have
+    # 2. Compute what we want
+    # 3. Create symbolic links if they're not there already
+    extensions_on_disk = fs.find(candidate_spec.prefix, expected['glob'])
+    link_names = [re.sub(expected['re'], ext_suffix,  s) for s in extensions_on_disk]
+    for file_name, link_name in zip(extensions_on_disk, link_names):
+        if os.path.exists(link_name):
+            continue
+        os.symlink(file_name, link_name)
+
+
 @_bootstrapper(type='buildcache')
 class _BuildcacheBootstrapper(object):
     """Install the software needed during bootstrapping from a buildcache."""