Revert "lit: revert 134b103"

This reverts commit d319005.

Causing messages like:

  File "...\Python\Python39\lib\multiprocessing\connection.py", line 816, in _exhaustive_wait
    res = _winapi.WaitForMultipleObjects(L, False, timeout)
ValueError: need at most 63 handles, got a sequence of length 74

llvm/utils/lit/lit/util.py

@@ -119,6 +119,12 @@ def usable_core_count():
         n = len(os.sched_getaffinity(0))
     except AttributeError:
         n = os.cpu_count() or 1
+
+    # On Windows, with more than 32 processes, process creation often fails with
+    # "Too many open files".  FIXME: Check if there's a better fix.
+    if platform.system() == 'Windows':
+        return min(n, 32)
+
     return n