SystemTools: Fix FilesDiffer 32-bit signed integer overflow on Windows

Markus87requested to merge
Markus87/kwsys:FixCopyIfDifferentWindowsBug into master

Otherwise it was wrong for different files of equal size bigger than 2GB on Windows.

Issue: cmake/cmake#23052 (closed)
Topic-rename: SystemTools-FilesDiffer-int-overflow

Edited by Brad King

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