[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:

    1. Adding missing options and commands.
    2. Standardising on double dashes for long-options throughout.
    3. Moving Mach-O specific options to a separate section.
    4. 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

llvm-svn: 364019