[ELF] Handle copy relocations for PC32.

This will eventually need to be refactored to better handle COPY relocations,
as other relocations can also generate them. I'm not yet sure the exact
circumstances in which they are needed yet.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@191567 91177308-0d34-0410-b5e6-96231b3b80d8