rtld: Use uintptr_t instead of Elf_Addr for init/fini function pointers

[jhb]

Summary

This is a no-op on non-CHERI architectures, but is required for CHERI
where Elf_Addr is only an address and not a complete pointer.

While here, consistently use uintptr_t * for arrays of init/fini
function pointers.

Effort: CHERI upstreaming
Obtained from: CheriBSD
Sponsored by: AFRL, DARPA

[imp]

This is a nice cleanup and eliminates a void * -> void ** type smearing we had with ElfAddr and should be a nop if I read right.