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