Build toolchains (mirrored)

Mirrors of musl.cc's aarch64-linux-musl-cross and x86_64-linux-musl-cross toolchains, used by build-scripts/build-vendor-deps.sh to cross-compile the droidspaces binary. Hosted here because musl.cc (not behind a CDN) proved unreliable from GitHub Actions runners -- a build failed after 12+ minutes of retried connection timeouts trying to reach it directly. Re-download from https://musl.cc if these ever need updating; not modified in any way, just re-hosted.