Change DT_* value definitions to macros in a separate file

Summary:
I recently added a new dynamic tag to our fork of LLVM and when adding it
to llvm-readobj I noticed that not all DT_ values were being handled there.

Using macros in a .def file that can be included by both ELFDumper.cpp and
the ELF.h header ensures that the two don't get out of sync when new values
are added.

Reviewers: grimar, pcc, davide, espindola

Reviewed By: grimar, espindola

Subscribers: srhines, llvm-commits

Differential Revision: https://reviews.llvm.org/D44558

llvm-svn: 328099