Two small cleanups for ELF's templates.

* We only ever specialize these templates with an instantiation of ELFType,
  so we don't need a template template.
* Replace LLVM_ELF_COMMA with just passing the individual parameters to the
  macro. This requires a second macro for when we only have ELFT, but that
  is still a small win.

llvm-svn: 179726