Split ELF relocation defintions into per-architecture .def files

This should allow the list of relocations for a particular
architecture to be kept in a single header rather than duplicated
whenever we need to enumerate all the relocations.

Patch by Will Newton.
Remove duplication of relocation names in lib/Object/ELF.cpp

We can now use the ELF relocation .def files to create the mapping
of relocation numbers to names and avoid having to duplicate the
list of relocations.

Patch by Will Newton.
Remove duplication of relocation names in lib/Object/ELFYAML.cpp

We can now use the ELF relocation .def files to create the mapping
of relocation numbers to names and avoid having to duplicate the
list of relocations.

Patch by Will Newton.