[2.1] compat: add check for kernel_neon_* availability

This patch backports the change from #15711 (to ZFS 2.2.x) because
some OS vendors have back-ported this change from Linux 6.2+ to their
older kernels.

The original patch addressed these issues:

1. Linux 6.2+ on arm64 has exported them with EXPORT_SYMBOL_GPL, so license compatibility must be checked before use.
2. On both arm and arm64, the definitions of these symbols are guarded by CONFIG_KERNEL_MODE_NEON, but their declarations are still present. Checking in configuration phase only leads to MODPOST errors (undefined references).

Signed-off-by: Keith Gable <keith@ziggythehamster.sh>
Co-authored-by: Shengqi Chen <harry-chen@outlook.com>