Ensure that Elf_Rel addends are always written for dynamic relocations

Summary:
This follows up on r321889 where writing of Elf_Rel addends was partially
moved to RelocationBaseSection. This patch ensures that the addends are
always written to the output section when a input section uses RELA but the
output is REL.

Differential Revision: https://reviews.llvm.org/D42843

llvm-svn: 325328