stick to NEON when building GROMACS on ARM CPUs, even when SVE is supported

[boegel]

(created using eb --new-pr)
sticking to NEON is known to lead to significantly better performance, see https://manual.gromacs.org/documentation/current/install-guide/index.html#nvidia-grace + https://gitlab.com/gromacs/gromacs/-/issues/5284 + https://gitlab.com/gromacs/gromacs/-/issues/5390 for more details

edit: draft, because this needs some more homework:

• changes to easyblock are totally untested;
• GROMACS issue only talks about Neoverse V2 (like NVIDIA Grace), what about Neoverse V1 which also supports SVE?

[migueldiascosta]

Note that the performance advantage of NEON over SVE, besides probably being specific to Grace, is specific to the short-range non bonded interactions, and "when the short-range non bonded interactions run on the GPU it is recommended to stay with GMX_SIMD=ARM_SVE" (https://manual.gromacs.org/documentation/current/install-guide/index.html#nvidia-grace), so the condition to use NEON should probably be for Grace plus non-CUDA builds, although it still depends on how GROMACs is run

To be honest, I think the main concern is to avoid ever using SVE with LLVM 20 (https://gitlab.com/gromacs/gromacs/-/issues/5390#note_2702855160), that's a ~10x performance regression. The NEON vs SVE difference reported for Grace is only ~10% (https://gitlab.com/gromacs/gromacs/-/issues/5284#summary), and that may change in the future, we wouldn't want to be accidentally stuck with NEON when that happens