AutoGen: Fix regression in timestamps on multi-config generators

Since !8564 (merged) 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 (closed)
Topic-rename: autogen-timestamp-per-config