bazel: split wee8 prebuilts by stdlib and select the correct archive for gcc

[Copilot]

//v8:wee8_smoke_test was consuming a libc++-built prebuilt archive on gcc/libstdc++ legs because wee8 prebuilts were keyed only by cpu/os. This changes wee8 packaging and selection to treat stdlib as part of the artifact identity, while keeping the existing libc++ artifact name stable.

• What changes

  • //v8:wee8_package_linux_x86_64 now produces both x86_64 variants in a single build via a Starlark transition:
    • v8-wee8-<version>-linux-x86_64.tar.xz (existing libc++ artifact, preserved)
    • v8-wee8-<version>-linux-x86_64-libstdcxx.tar.xz
  • aarch64 remains libc++ only and keeps the existing artifact name.

• Packaging / transition

  • Adds an outgoing transition in bazel/v8/wee8_package.bzl that flips //compile:use_gcc_toolchain for the libstdc++ package edge.
  • The transition also sets //command_line_option:extra_toolchains, so the gcc toolchains are selected within the transitioned configuration; no extra CI matrix leg is needed.

• Prebuilt repository layout

  • setup_wee8_prebuilt now instantiates:
    • @wee8_prebuilt_x86_64
    • @wee8_prebuilt_x86_64_libstdcxx
    • @wee8_prebuilt_aarch64
  • VERSIONS["wee8_sha256"] and the release update logic are keyed by both arch and stdlib.
  • Missing prebuilt hashes no longer generate a text file named lib/libwee8.a; the repo instead exposes an explicitly incompatible target with a clear message.

• Consumer selection

  • bazel/v8/BUILD now defines //v8:wee8 locally and selects on both platform and //compile:use_gcc_toolchain.
  • On linux/x86_64:
    • default config resolves to the libc++ prebuilt
    • --config=gcc resolves to the libstdc++ prebuilt
  • On linux/aarch64 + gcc:
    • selection fails explicitly instead of silently falling through to the libc++ archive

• Module / workspace wiring

  • Wires wee8 prebuilt repos into both WORKSPACE and MODULE.bazel so the same prebuilt selection model exists in workspace and bzlmod modes.
  • Adds a small bzlmod extension for fixed-name wee8 prebuilt repos.

• CI / release metadata

  • Updates package-v8 artifact globs to upload the new stdlib-qualified tarball alongside the existing libc++ artifact.
  • Raises --experimental_ui_max_stdouterr_bytes in the Bazel CI invocation for diagnosable link failures.
  • Updates release SHA extraction to understand the new artifact naming scheme.

Example of the new selection shape:

alias(
    name = "wee8",
    actual = select({
        ":linux_x86_64_gcc": "@wee8_prebuilt_x86_64_libstdcxx//:wee8",
        ":linux_x86_64": "@wee8_prebuilt_x86_64//:wee8",
        ":linux_aarch64_gcc": ":wee8_aarch64_gcc_unsupported",
        ":linux_aarch64": "@wee8_prebuilt_aarch64//:wee8",
        "//conditions:default": "@v8//:wee8",
    }),
)

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