Update Spacemit toolchain source and centralize versioning

[Copilot]

The riscv64 Spacemit build path still referenced the legacy GCC toolchain host in both the local build script and CI workflow. This change switches those downloads to the GitHub Releases URL and derives the archive name/URL from a single version variable in each location.

• Build script

  • Replaced the old archive.spacemit.com toolchain URL in build-riscv64-linux-gnu-spacemit.sh
  • Introduced SPACEMIT_TOOLCHAIN_VERSION as the local source of truth
  • Derived both the toolchain directory name and downloaded archive path from that version

• GitHub Actions workflow

  • Replaced the hard-coded toolchain cache/download URL in .github/workflows/riscv64-spacemit-linux.yaml
  • Added a job-level SPACEMIT_TOOLCHAIN_VERSION
  • Computed the archive name and release URL from the version in the download step
  • Updated the cache key to track the new GitHub-hosted artifact

• Versioning cleanup

  • Removed duplicated embedded version strings where the same archive metadata was repeated
  • Reduced future toolchain bumps to a single version edit per file

SPACEMIT_TOOLCHAIN_VERSION=v1.1.2
SPACEMIT_TOOLCHAIN=spacemit-toolchain-linux-glibc-x86_64-${SPACEMIT_TOOLCHAIN_VERSION}
DOWNLOAD_URL="https://github.com/spacemit-com/toolchain/releases/download/${SPACEMIT_TOOLCHAIN_VERSION}/${SPACEMIT_TOOLCHAIN}.tar.xz"