[ELF] Customize a relocation table output format (rel / rela).
Add new virtual virtual function `isRelaOutputFormat` to the
`ELFLinkingContext` class. Call this function everywhere we need to
select a relocation table format.

Patch reviewed by Shankar Easwaran and Rui Ueyama.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@199973 91177308-0d34-0410-b5e6-96231b3b80d8