[llvm-readobj] Little clean up inside `parseDynamicTable`

Summary:
This anoymous function actually has same logic with `Obj->toMappedAddr`.

Besides, I have a question on resolving illegal value. `gnu-readelf`, `gnu-objdump` and `llvm-objdump` could parse the test file 'test/tools/llvm-objdump/Inputs/private-headers-x86_64.elf', but `llvm-readobj` will fail when parse `DT_RELR` segment. Because, the value is 0x87654321 which is illegal. So, shall we do this clean up rather then remove the checking statements inside anoymous function?

```
if (Delta >= Phdr.p_filesz)
    return createError("Virtual address is not in any segment");
```

Reviewers: rupprecht, jhenderson

Reviewed By: jhenderson

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D55329

llvm-svn: 348701