[ELF] - Reset output section size when assigning offsets.

In many places we reset Size to 0 before calling assignOffsets()
manually. Sometimes we don't do that. 
It looks we can just always do that inside.

Previous code had:
template <class ELFT> void OutputSection::assignOffsets() {
  uint64_t Off = Size;

And tests feels fine with Off = 0. 
I think Off = Size make no sence.

Differential revision: https://reviews.llvm.org/D30463

llvm-svn: 296609