When running check-generated-files in update mode, all generated files were
regenerated. As a consequence,
```
tests/scripts/check-generated-files.sh -u && make
```
always caused most of the code to be rebuilt. Now, if a file hasn't changed,
preserve its original modification time (and other metadata), so the command
above doesn't rebuild anything that has actually not changed.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
b5c4382bef