When using Rela, don't write the addend to the output section.

The Elf_Rela has an explicit addend. It doesn't need the addend to be
written to the section being relocated.

Since relative relocations are very common in the output, this is a
noticeable speedup. The results I got were

chromium
  master 4.778149487
  patch  4.761120792 0.996436131802
chromium fast
  master 1.896253636
  patch  1.840990582 0.970856718241
the gold plugin
  master 0.399337811
  patch  0.392279276 0.982324401032
clang
  master 0.666873675
  patch  0.665895708 0.998533504865
llvm-as
  master 0.037101095
  patch  0.037123149 1.00059442989
the gold plugin fsds
  master 0.422473396
  patch  0.414192879 0.980399909016
clang fsds
  master 0.747302008
  patch  0.744843964 0.996710775599
llvm-as fsds
  master 0.033146245
  patch  0.033064531 0.997534743377
scylla
  master 4.08857525
  patch  4.082245184 0.998451767275

llvm-svn: 269417