Fix mingw_w64_pthreads: rebuild with working cross-stdenv

nixpkgs-2511's crossThreadsStdenv resolves to native Linux stdenv
instead of the cross-compiling stdenv, causing dll_dependency.S to be
assembled with the 64-bit Linux assembler which rejects 32-bit Windows
push/pop instructions. Rebuild from the same mingw-w64 source using
prev.stdenv (the full Windows cross-stdenv) which properly provides
the cross-compiler. No circular dependency because GCC uses MCF
threads (depends on mcfgthreads, not pthreads).

Author: angerman

flake.nix

@@ -122,6 +122,16 @@
            # but the package only creates a dlltool wrapper, not windres.
            # The prefixed x86_64-w64-mingw32-windres exists in the cross-bintools
            # but meson searches for the unprefixed name. Same pattern as dlltool.
+           #
+           # mingw_w64_pthreads: nixpkgs-2511's crossThreadsStdenv resolves to
+           # native Linux stdenv instead of the cross-compiling stdenv, causing
+           # dll_dependency.S to be assembled with the 64-bit Linux assembler
+           # which rejects 32-bit Windows push/pop instructions. The properly
+           # cross-compiled version exists in nixpkgs (built via crossLibcStdenv)
+           # but windows.mingw_w64_pthreads uses the broken crossThreadsStdenv.
+           # Fix: rebuild from the same mingw-w64 source using the working
+           # cross-stdenv (prev.windows.mingw_w64 uses crossLibcStdenv which
+           # properly provides the cross-compiler).
            windows = prev.windows // {
              mcfgthreads = prev.windows.mcfgthreads.overrideAttrs (old: {
                nativeBuildInputs = old.nativeBuildInputs ++ [
@@ -130,6 +140,14 @@
                  '')
                ];
              });
+             mingw_w64_pthreads = prev.stdenv.mkDerivation {
+               name = "${prev.windows.mingw_w64.name}-pthreads";
+               inherit (prev.windows.mingw_w64) src meta;
+               configureFlags = [ "--enable-static" ];
+               preConfigure = ''
+                 cd mingw-w64-libraries/winpthreads
+               '';
+             };
            };
          });