Merge pull request #66104 from tristanlabelle/support-no-NUMBER_OF_PROCESSORS

Support NUMBER_OF_PROCESSORS not being defined when running tests on Windows

Author: compnerd

test/lit.cfg

@@ -1447,7 +1447,8 @@ if run_vendor == 'apple':
 
 elif run_os in ['windows-msvc']:
     lit_config.note('Testing Windows ' + config.variant_triple)
-    config.environment['NUMBER_OF_PROCESSORS'] = os.environ['NUMBER_OF_PROCESSORS']
+    if 'NUMBER_OF_PROCESSORS' in os.environ:
+        config.environment['NUMBER_OF_PROCESSORS'] = os.environ['NUMBER_OF_PROCESSORS']
     if 'PROCESSOR_ARCHITEW6432' in os.environ:
         config.environment['PROCESSOR_ARCHITEW6432'] = os.environ['PROCESSOR_ARCHITEW6432']
     if 'PROCESSOR_ARCHITECTURE' in os.environ: