Fix GCC 8/9/10 nightly failures: bit_cast, brace-init, -Wno-gnu-*

[alliepiper]

Fixes three independent build failures from the 2026-05-15 nightly (run 25900203790).

Changes

• libcudacxx bit_cast.h: GCC 8/9/10 don't register __float128 in standard type traits, so default_initializable<__float128> fails concept check even though __float128 x; is valid C++. The __builtin_bit_cast path is available from GCC 11+; extend the existing GCC ≤8 workaround to GCC ≤10.

• CUB catch2_test_device_reduce_env.cu: GCC 8/9 require an explicit type name when returning an aggregate via braced-init-list. Fix two callsites (nondeterministic_reduce_tuning and deterministic_reduce_tuning) to use named constructors instead of bare {...}.

• CMake CCCLBuildCompilerTargets.cmake: -Wno-gnu-line-marker and -Wno-gnu-zero-variadic-macro-arguments are silently accepted by GCC < 10 without -Werror, so append_option_if_available adds them; but the actual build has -Werror active and passes these via nvcc -Xcompiler=, where GCC < 10 rejects unknown -Wno-* flags. Guard both flags behind GCC >= 10.

Test plan

• GCC 10 + CTK 12.0: numeric_limits<__float128>::infinity() precompile — ✅ PASS
• GCC 11 + CTK 12.0: same, no regression — ✅ PASS
• GCC 8 + CTK 12.0: cub.test.device.reduce_env.lid_0 build — ✅ PASS
• GCC 9 + CTK 12.0: same — ✅ PASS

Notes

CI is scoped to the affected GCC 8/9/10 toolchains via workflows.override. The override commit and skip tags must be reset before merging.

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[alliepiper]

/ok to test 1c6c2cf

[miscco]

wasnt there a PR that changed this to is_constructible

[alliepiper]

We switched it back (@fbusato) 8463734

I just noticed that the static assert message doesn't match the comparison, I'll fix that here.

[alliepiper]

/ok to test e91bd7b

[github-actions]

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