elf.h: Add BPF relocation types.

The BPF ELF format has new relocation types R_BPF_64_64 and R_BPF_64_32.
The existing R_BPF_MAP_FD was an extension that never got implemented.
Remove it, because the constant conflicts with the official R_BPF_64_64.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>