simd_stat: fix undefined CONFIG_KERNEL_MODE_NEON error on armel

CONFIG_KERNEL_MODE_NEON depends on CONFIG_NEON. Neither is defined
on armel. Add a guard to avoid compilation errors.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Shengqi Chen <harry-chen@outlook.com>
Closes #16871