Add a MachO-specific "mode" to llvm-objdump, that, if enabled, gathers...

Add a MachO-specific "mode" to llvm-objdump, that, if enabled, gathers additional information that are only available on MachO.

- It can take FunctionStarts from a binary to find entry points more accurately.
- Symbol offsets in executables are correct now.

llvm-svn: 140028