Symbol cpu_feature_keys should be exported to all modules on powerpc

[王昊然]

Just like symbol 'mmu_feature_keys'[1], 'cpu_feature_keys' was referenced
indirectly by many inline functions; any GPL-incompatible modules using such
a function will be potentially broken due to 'cpu_feature_keys' being
exported as GPL-only.

For example it still breaks ZFS, see
https://github.com/openzfs/zfs/issues/14545

[1]: https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20220329085709.4132729-1-haokexin@gmail.com/