[ELF] Delete relOff from TargetInfo::writePLT

This change only affects EM_386. relOff can be computed from `index`
easily, so it is unnecessarily passed as a parameter.

Both in.plt and in.iplt entries are written by writePLT. For in.iplt,
the instruction `push reloc_offset` will change because `index` is now
different. Fortunately, this does not matter because `push; jmp` is only
used by PLT. IPLT does not need the code sequence.

Reviewed By: grimar, ruiu

Differential Revision: https://reviews.llvm.org/D71518