Add "-format darwin" to llvm-nm to be like darwin's nm(1) -m output.

This is a first step in seeing if it is possible to make llvm-nm produce
the same output as darwin's nm(1).  Darwin's default format is bsd but its
-m output prints the longer Mach-O specific details.  For now I added the
"-format darwin" to do this (whos name may need to change in the future).
As there are other Mach-O specific flags to nm(1) which I'm hoping to add some
how in the future.  But I wanted to see if I could get the correct output for
-m flag using llvm-nm and the libObject interfaces.

I got this working but would love to hear what others think about this approach
to getting object/format specific details printed with llvm-nm.