AutoGen: Fix regression in timestamps on multi-config generators
Since commit fddd0f0443 (Autogen: AUTO*_EXECUTABLE: add support for
per-config values, 2023-06-14) we do not correctly generate outputs
for one configuration after another configuration has been built.
Fix this:
- Revert some config based stuff for `Xcode` due to the `$<CONFIG>`
genex usage limitation in source files with `Xcode`.
- For multi-config generators use a per-config `timestamp_$<CONFIG>`
file instead of one `timestamp` file.
Fixes: #25261
3 files changed
