[docs][llvm-objdump] Improve llvm-objdump documentation
The llvm-objdump document was missing many options, and there were also
some style issues with it. This patches fixes all but the first issue
listed in https://bugs.llvm.org/show_bug.cgi?id=42249 by:
- Adding missing options and commands.
- Standardising on double dashes for long-options throughout.
- Moving Mach-O specific options to a separate section.
- Removing options that don't exist or aren't relevant to llvm-objdump.
Reviewed by: MaskRay, mtrent, alexshap
Differential Revision: https://reviews.llvm.org/D63606