[llvm-objdump] Add testing for --print-imm-hex, --headers, --section-headers and --private-headers

llvm-objdump had some missing coverage that is fixed by this change:
- A test specifically for --print-imm-hex, and coverage of --no-print-imm-hex
- section-headers.test checks the aliases --headers or --section-headers
- A test for the use of --private-headers for ELF that checks the output
- A test for ELF program headers

Differential Revision: https://reviews.llvm.org/D103974