Use elf_relocaddr() when handling R_X86_64_RELATIVE relocations.

[markj]

Summary

This is necessary for accesses to DPCPU and VNET data variables in
PIC KLDs. This should have no functional impact for non-PIC KLDs since
such code shouldn't contain R_X86_64_RELATIVE relocations.

[kib]

How hard would be assert that lf is a dso, as opposed to an object file ?

[markj]

In D21755#474725, @kib wrote:

How hard would be assert that lf is a dso, as opposed to an object file ?

It would be a bit of work: there is no field in the base linker_file which tells you the type of the file. It is of course trivial to add one though, so perhaps I should just do that and add the assert to elf_relocaddr().

[markj]

Add an assertion to elf_relocaddr(). This is a bit hacky but
hacky in the same way as linker_add_class().