[llvm-objdump --macho] Rename --dyld_info to --dyld-info

llvm-objdump takes foo-bar style flags, while llvm-otool takes foo_bar style
flags.  dyld_info was the only exception to that.

Add a -dyld_info flag to llvm-otool instead.

(Both in llvm-objdump and llvm-otool, the flag doesn't really do anything
yet.)

Differential Revision: https://reviews.llvm.org/D131897