Symbol cpu_feature_keys should be exported to all modules on powerpc

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/20220329085709.41327...