Enable SIMD

[ax3l]

For our CPU builds of ImpactX 25.08+, it should be safe to enable SIMD if we set an march (and mtune) that is old enough.

Tasks:

• Enable ImpactX_SIMD and pick an old enough march to support (e.g. SSE3 but no AVX) -- check what conda-forge does. Last update was 2017 and picked to support:
  • Linux x86_64: -march=nocona -mtune=haswell (2004+)
  • Linux x86: ...
  • macOS x86_64: -march=core2 -mtune=haswell -mssse3
  • macOS arm64:
  • Windows x86_64:
  • Windows x86:

[ax3l]

The 25.09+ release series could add this here, as it ships a few bug fixes, especially for macOS/Windows.