ci: use POSIX threads for Nix Windows builds #36316

Guix release binaries use GCC’s POSIX thread model, while the pinned Nixpkgs toolchain defaults to mcf. This makes POSIX the default for Nix Windows cross builds. Nightly builds can select mcf or win32 with the threadModel argument.

The Nixpkgs threads setting selects the model before GCC is built. Only the mcf model pulls in mcfgthreads. For POSIX, GCC’s target libraries need winpthreads headers and libraries during the compiler build. The EXTRA_*_FOR_TARGET flags provide those paths without exposing target headers to GCC’s native build tools, which caused the earlier build failure.

The compiler wrapper includes winpthreads through extraPackages, so the shell no longer needs to export pthread include and library flags manually. CI also prints GCC’s configured thread model to make mismatches visible.

Fixes #36314

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36316.

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Added a second commit we may want to drop to print/verify first in the CI run.

Concept ACK.

3edcc510370c7fd804060960a0a5e16bbe356c96

Could we make the thread model configurable? This would help with testing MCF in nightly builds.

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🚧 At least one of the CI tasks failed. _{Task lint: https://github.com/bitcoin/bitcoin/actions/runs/35753251918/job/106832510905} _{LLM reason (✨ experimental): CI failed because the shell lint step (shellcheck) reported SC2016 in ci/test/01_base_install.sh line 55 (expressions inside single quotes).}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

</details>

Could we make the thread model configurable? This would help with testing MCF in nightly builds.

I’ve made the thread model configurable through a Nix argument. e.g. to test mcf:

HOST=x86_64-w64-mingw32ucrt nix-shell contrib/devtools/shell-win64-cross.nix --argstr threadModel mcf