Enable back parallel sync on cygwin.

gclient.py

Index: gclient.py
diff --git a/gclient.py b/gclient.py
index 30bad034d775d8bfb1b5e67d30e0dc92c65a1534..9d920f91bb4dec729ab275ee4479941de8040eb7 100644
--- a/gclient.py
+++ b/gclient.py
@@ -1515,8 +1515,8 @@ def GenUsage(parser, command):
 def Parser():
   """Returns the default parser."""
   parser = optparse.OptionParser(version='%prog ' + __version__)
-  # cygwin and some arm boards have issues with parallel sync.
-  if sys.platform == 'cygwin' or platform.machine().startswith('arm'):
+  # some arm boards have issues with parallel sync.
+  if platform.machine().startswith('arm'):
     jobs = 1
   else:
     jobs = 8