[lld][ELF][All Archs] Addend is used by dynamic relocations

only if they are relative. This removes the FIXME when the
relocations are being emitted and checks if the relocation
is relative and only then populates the addend information.

I couldnt add a testcase for this as llvm-readobj lacks     
functionality of printing dynamic relocations.              

When the functionality is added, remove the commented lines
from elf/ifunc.test to test functionality.

llvm-svn: 182077