ios: Use _chsize_s on Windows for file truncation (#24616)
Should allow files >2 GB to be truncated on Windows.
Fix #24466
Checking Windows 32bit build https://julialang-s3.julialang.org/bin/winnt/x86/0.6/julia-0.6.1-win32.exe, libgit2.dll already uses `_chsize_s`, so both 32bit and 64bit should be able to use `_chsize_s`.
