In r268055, powerpc64 was special cased regarding linker sets to not mark the function pointer as 'const'.
This appears to have been done to ensure the compiler generates the function descriptors correctly. When non-const, the function descriptors will end up in the .data.rel.local section, and the linker set symbols will get relocations pointing to them there.
Since function pointers on ELFv2 are "just" pointers like other platforms, we can leave them const like they are on every other platform.