ELF: Remove TargetHandlerBase by merging it with TargetHandler.

In r233772, I removed an empty class, DefaultTargetHandler, from
the class hierarchy by merging the class with TargetHandler. I then
found that TargetHandler and its base class, TargetHandlerBase,
are also almost the same.

We need to go deeper.

In this patch, I merged TargetHandlerBase with TargetHandler.
The only difference between them is the existence (or absense)
of a pure virtual function registerRelocationName(). I added that
function to the (new) TargetHandler.

One more thing is that TargetHandler was templated for no reason.
I made it non-templated class.

llvm-svn: 233773