Fix this config name for consistency with non-bootstrap platform config.
This also fixes a build failure of the bootstrap tools on
aarch64-darwin, where a test in LLVM 21 assumes it's spelt arm64, not
aarch64.
Also saves on having to rebuild LLVM just for bootstrap tools, now that
the config is consistent.
- update the hashes and tools needed to extract the bootstrap-tools archive
- unify the x64 and aarch64 unpack process
- unpin libxml for python-minimal