This PR fixes two issues with Lake's process of creating static
archives.
Lake now always recreates static archives by first deleting any existing
one and then recreating it. `ar rcs` does not remove delete files, so
running it when the archive already exists can leave behind "ghost"
symbols of removed object files.
Second, Lake now use `T` rather than `--thin` to create thin archives.
While `--thin` is the recommended spelling, older versions of LLVM `ar`
do not support it. Thus, either choice produces tradeoffs. `T` is chosen
to make Lake consistent with the Lean core's own (Make) build scripts.
6f85e32501