[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