stage1: don't copy unchanged output files

when both `--cache on` and `--output-dir` parameters
are provided. This prevents re-linking `zig` with every
`make` even when `libzigstage2.a` was unchanged.
a26800c099Andrew Kelley committed on 2/17/2020, 2:10:03 AMparent5c54d7b
4 files changedLine totals unavailable