[Microsoft.Build] Fix XS build error

[Microsoft.Build] Fix XS build error

We observe the following error when building XS:

"Threadpool worker" tid=0x0x7f011bfff700 this=0x0x7f01394bfb30 thread handle 0x432 state : not waiting owns ()
  at <unknown> <0xffffffff>
  at (wrapper managed-to-native) System.Threading.WaitHandle.WaitAll_internal (System.Threading.WaitHandle[],int,bool) <0x0005d>
  at System.Threading.WaitHandle.WaitAll (System.Threading.WaitHandle[]) <0x00026>
  at Microsoft.Build.Utilities.ProcessWrapper.<Start>m__0 (object,System.EventArgs) <0x0008f>
  at System.Diagnostics.Process.OnExited () <0x000ed>
  at System.Diagnostics.Process.RaiseOnExited () <0x0009f>
  at System.Diagnostics.Process.CompletionCallback (object,bool) <0x00017>
  at System.Threading.RegisteredWaitHandle.DoCallBack (object) <0x00088>
  at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context (object) <0x00058>
  at System.Threading.ExecutionContext.RunInternal (System.Threading.ExecutionContext,System.Threading.ContextCallback,object,bool) <0x001c6>
  at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext,System.Threading.ContextCallback,object,bool) <0x00020>
  at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem () <0x0004c>
  at System.Threading.ThreadPoolWorkQueue.Dispatch () <0x001d6>
  at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback () <0x00008>
  at (wrapper runtime-invoke) <Module>.runtime_invoke_bool (object,intptr,intptr,intptr) <0x0005a>

And the process output would contains a bunch of "_wapi_handle_ref: Attempting to ref unused handle 0x1234" and "_wapi_handle_unref_full: Attempting to unref unused handle 0x1234".

The expected behaviour is for WaitHandle.(WaitAll|WaitAny|WaitOne|SignalAndWait) and EventWaitHandle.Set to throw an ObjectDisposedException in case the WaitHandle (or one of them if calling WaitAll, WaitAny or SignalAndWait) have been disposed.

This error would be due to a race between ProcessWrapper.Dispose, which closes endEventOut and endEventErr, and the above Process.Wrapper.<Start>m__0 (the base.Exited) callback, which waits on endEventOut and endEventErr.