[Copilot]

Layers review follow-ups onto copilot/gcc-toolchain-hermetic-fix, retaining hermetic GCC 13.4.0, a shared base sysroot, and static C++ runtimes. Preserves downstream compatibility while providing a migration path away from libstdc++ sysroot variants.

• Legacy sysroots: Restore stdcc_version, published hashes, and download URLs. Pin deprecated variants to their existing release; preserve release metadata without rebuilding them.
• Runtime exports: Add public GCC :libstdcxx and :libgcc targets, CcInfo analysis coverage, and prefixer migration guidance:

  copts = ["-nostdinc++", "-stdlib=libstdc++"],
  deps = ["@gcc_linux_x86_64//:libstdcxx"],

• Registration and execution: Make internal GCC registrations dev-only. Remove the unusable arm64 GCC execution platform; aarch64 GCC builds cross-compile on x86_64 workers.
• ABI enforcement: Centralize GCC pins, add static-runtime compatibility checks, and document measured GLIBC requirements and compiler search paths. Add a linked wee8 probe to GCC CI, retaining archive-name checks as a pre-filter.

Release dependency: Rebuild both wee8 libstdc++ archives and publish their hashes in the next bins-v* release. The linked probe remains hash-gated until the incompatible x86_64 prebuilt is replaced; aarch64 awaits its first archive.