Clarify getRelocationAddress x getRelocationOffset a bit.

getRelocationAddress is for dynamic libraries and executables,
getRelocationOffset for relocatable objects.

Mark the getRelocationAddress of COFF and MachO as not implemented yet. Add a
test of ELF's. llvm-readobj -r now prints the same values as readelf -r.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180259 91177308-0d34-0410-b5e6-96231b3b80d8