Changes for Emscripten 6 compilation and upcoming Pyodide ABI support

[agriyakhetarpal]

The next Pyodide ABI (2026_5) will be based on Emscripten 6.0.3 and CPython 3.15.

TODO items:

• Port @hoodmane's patch to fix BLIS's builds with Emscripten 6
• Open a PR to update the current downstream C-only testing in semicolon-lapack, to test against both Emscripten 5.0.3 and Emscripten 6.0.3
• Update Compile a Fortran-free WASM SciPy, with BLIS and semicolon-lapack pyodide-recipes#619 and fix builds and tests as necessary
• Update SciPy when the ABI is finalised
  • Test against nightly Pyodide xbuildenvs (or, better, a Pyodide 315.0 alpha release) on my fork of SciPy beforehand
  • Add 315 alpha to cibuildwheel

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.