ELF: Move registerRelocationNames() from TargetHandler to <Arch>ELFLinkingContext.

registerRelocationNames() function is called to register all known
ELF relocation types to the central registry. Since we have separate
LinkingContext class for each ELF machine type, we need to call the
function for each LinkingContext.

However, the function belonged to TargetHandler instead of LinkingContext.
So we needed to do ctx.getTargetHandler().registerRelocationNames().
This patch removes that redundant getTargetHandler call by moving the
function from TargetHandler to LinkingContext.

Conceptually this patch is small, but in reality it's not that small.
It's because the same code is copied to each architecture.
Most of this patch is just repetition. We need to merge them, but
that cannot be done in this patch.

llvm-svn: 233883