[llvm-objdump] Add an llvm-otool tool

This implements an LLVM tool that's flag- and output-compatible
with macOS's `otool` -- except for bugs, but from testing with both
`otool` and `xcrun otool-classic`, llvm-otool matches vanilla
otool's behavior very well already. It's not 100% perfect, but
it's a very solid start.

This uses the same approach as llvm-objcopy: llvm-objdump uses
a different OptTable when it's invoked as llvm-otool. This
is possible thanks to D100433.

Differential Revision: https://reviews.llvm.org/D100583