[compiler-rt] Provide better llvm-lit failure when llvm-config fails.

[hctim]

Summary

The current error message can cause confusion if llvm-config can't be
executed for reasons other than "not found". In my example, cross
compiling generated an llvm-config binary for aarch64 which couldn't be
executed natively. Instead of telling me that the error was with the
file architecture, it reports the file as not being present.

[hctim]

• Fixed a minor merge error with this patch.

[eugenis]

LGTM

compiler-rt/test/lit.common.cfg

if config.have_rpc_xdr_h:
   config.available_features.add('sunrpc')
 
 # Ask llvm-config about assertion mode.
 try:
   llvm_config_cmd = subprocess.Popen(
       [os.path.join(config.llvm_tools_dir, 'llvm-config'), '--assertion-mode'],
       stdout = subprocess.PIPE,
       env=config.environment)
-except OSError:
-  print("Could not find llvm-config in " + config.llvm_tools_dir)
+except OSError as e:
+  print("Could not launch llvm-config in " + config.llvm_tools_dir)
+  print("    Failed with error #{0}: {1}".format(e.errno, e.strerror))
   exit(42)
 
 if re.search(r'ON', llvm_config_cmd.stdout.read().decode('ascii')):
   config.available_features.add('asserts')
 llvm_config_cmd.wait()
 
 # Sanitizer tests tend to be flaky on Windows due to PR24554, so add some
 # retries. We don't do this on otther platforms because it's slower.