[llvm-objdump] Adjust spacing and field width for --section-headers

Summary:
- Expand the "Name" column past 13 characters when any of the section names are longer. Current behavior is a staggard output instead of a nice table if a single name is longer.
- Only print the required number of hex chars for addresses (i.e. 8 characters for 32-bit, 16 characters for 64-bit)
- Fix trailing spaces

Reviewers: grimar, jhenderson, espindola

Reviewed By: grimar

Subscribers: emaste, sbc100, arichardson, aheejin, seiya, llvm-commits, MaskRay

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68730

llvm-svn: 374795