Remove unused code in bfd
Add (disabled) code to relax calls to PLT stubs

This is useful to have around, but instead of relaxing PLT calls from
auipc/jalr to jal, we relax them to a gp-relative load and a jalr.
This bypasses the PLT entirely (except for the very first call),
reducing dynamic instruction count (including a jump), at the cost of
an extra static instruction per PLT call.

We might revisit this decision.  Sometimes, saving static code size is
more important than BTB pressure or even spatial locality.
Fix incorrect relaxation fixups of non-branches

We shouldn't fix up relocations against symbols after deleting code, since
the symbols aren't computed until after that stage.