Merge pull request #2932 from ludovic-henry/fix-waithandle-remoting

[corlib] Ifdef out calls to SynchronizationAttribute on monotouch

When WaitHandle was imported from referencesource in f6fa8f0, the code was refactored and basically all WaitHandle.Wait* calls end up in WaitOneNative or WaitMultiple now.

On monotouch those methods threw a NotSupportedException when exitContext == true even before the import, so this behavior got inherited to some other methods that didn't throw after the exception was removed from them in 93e31cb. This broke tests in maccore and other code that passed exitContext=true.

To fix this, we just ifdef out the calls to SynchronizationAttribute on monotouch since they don't make sense there anyway since remoting isn't available. The commit that initially added the exception in 2ce9fc1 only did that to make the behavior consistent when the linker is enabled. By removing the calls we get the same result.

Do the same for Monitor so we're consistent.

commit message from @akoeplinger