[llvm-objdump] Should print unknown d_tag in hex format

Summary:
Currently, `llvm-objdump` prints "unknown" instead of d_tag value in hex format. Because getDynamicTagAsString returns "unknown" rather than empty 
string.

Reviewers: grimar, jhenderson

Reviewed By: jhenderson

Subscribers: rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58763

llvm-svn: 355262