Revert "[compiler-rt] Default to Lit's Internal Shell"

This reverts commit 16ab8c0.

It appears this broke a couple of buildbots:
1. https://lab.llvm.org/buildbot/#/builders/193/builds/11847
2. https://lab.llvm.org/buildbot/#/builders/161/builds/8736

Reverting for now so I have a chance to investigate.

Author: boomanaiden154

compiler-rt/test/lit.common.cfg.py

@@ -113,17 +113,12 @@ def push_dynamic_library_lookup_path(config, new_path):
         config.environment[dynamic_library_lookup_var] = new_ld_library_path_64
 
 
-# TODO: Consolidate the logic for turning on the internal shell by default for all LLVM test suites.
-# See https://github.com/llvm/llvm-project/issues/106636 for more details.
-#
 # Choose between lit's internal shell pipeline runner and a real shell.  If
 # LIT_USE_INTERNAL_SHELL is in the environment, we use that as an override.
-use_lit_shell = True
-lit_shell_env = os.environ.get("LIT_USE_INTERNAL_SHELL")
-if lit_shell_env:
-    use_lit_shell = lit.util.pythonize_bool(lit_shell_env)
+use_lit_shell = os.environ.get("LIT_USE_INTERNAL_SHELL")
 if use_lit_shell:
-    execute_external = True
+    # 0 is external, "" is default, and everything else is internal.
+    execute_external = use_lit_shell == "0"
 else:
     # Otherwise we default to internal on Windows and external elsewhere, as
     # bash on Windows is usually very slow.