[mypyc] librt base64: support pyodide via the NEON intrinsics

[mr-c]

Fixes the following error when building lib-rt with pyodide

/home/runner/work/librt/librt/.pyodide_build/pywasmcross_symlinks/cc -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall -O2 -g0 -fPIC -DPY_CALL_TRAMPOLINE -I. -Ibase64 -I/tmp/build-env-ftmn_jvi/include -I/home/runner/.cache/cibuildwheel/pyodide-build-0.30.7/0.27.7/xbuildenv/pyodide-root/cpython/installs/python-3.12.7/include/python3.12 -c base64/arch/avx/codec.c -o build/temp.emscripten_3_1_58_wasm32-cpython-312/base64/arch/avx/codec.o -O3 -mavx
emcc: error: passing any of -msse, -msse2, -msse3, -mssse3, -msse4.1, -msse4.2, -msse4, -mavx, -mfpu=neon flags also requires passing -msimd128 (or -mrelaxed-simd)!

Instead of using emscripten's compatibility headers for X86 intrinsics, this PR has pyodide/emscripten use the NEON intrinsics as they better match the underlying SIMD capabilities in WASM. And it skips the "CPU detection" which would not work (nor make sense) in a WASM setting.

Ideally a pure WASM implementation would be added to base64 and backported here as well.

[ilevkivskyi]

cc @JukkaL FYI

[ilevkivskyi]

Also cc @emmatyping who added the original WASM build

[ilevkivskyi]

Apart from ever increasing amount of code duplication this looks good to me. If there are no objections I am going to merge this today/tomorrow.

[emmatyping]

This looks good! Thanks for the fix :)