see dotnet/macios#4277 (comment)

Steps to Reproduce

1. build mono-2018-06 branch from xamarin/xamarin-macios
2. run FileTest.LastWriteTimeSubMsCopy 32-bit or 64-bit mode on an iOS 10.3.3 device

since it worked on 2018-04 (?) the possible candidates for this regression are:

• [2018-06] [MacOS] On Mac, use the copyfile API to copy files #9696
• [2018-06] [w32file] Fix usage of copyfile #10010 which only exists on 2018-06 if I see it correctly

potential missing piece:

• Use clonefile instead of copyfile on macOS. #10020 merged on master and 2018-08. It might should replace [2018-06] [w32file] Fix usage of copyfile #10010