Changes for Emscripten 6 compilation and upcoming Pyodide ABI support #1

The next Pyodide ABI (2026_5) will be based on Emscripten 6.0.5 and CPython 3.15.

TODO items:

• Port @hoodmane's patch to fix BLIS's builds with Emscripten 6 (Fix libblis: Feature detect Emscripten with __EMSCRIPTEN__ pyodide-recipes#630)
  • Done at ac8141f
• Open a PR to update the current downstream C-only testing in semicolon-lapack, to test against both Emscripten 5.0.3 and Emscripten 6.0.5
  • Done at CI/BLD: Build with Emscripten 6.0.5 ilayn/semicolon-lapack#34
• Update SciPy when the ABI is finalised
  • Build SciPy against Emscripten 6.0.5 matching a Pyodide 315.0 alpha release
    • Built at Emcc 6.0.5 pyodide-recipes#639
    • Run SciPy tests and fix any failing ones on Emcc 6.0.5 pyodide-recipes#639: I just added the scipy label there, so the build is in progress, and builds and tests to be fixed as necessary
• Add 315 alpha to cibuildwheel
  • Done at Add pyodide_315 support pypa/cibuildwheel#2958
• I just updated Compile a Fortran-free WASM SciPy, with BLIS and semicolon-lapack pyodide-recipes#619, so the build is in progress, and builds and tests to be fixed as necessary

Till the time both we support both the 2026_0 and 2026_5 ABIs, this new work will be hosted on a new pyodide-2.1-2026_5 branch, as opposed to the pyodide-2.1 branch.