Add the "-s" flag to llvm-nm for Mach-O files that prints symbols only in
the specified section.  This is same functionality as darwin’s nm(1) "-s" flag.

There is one FIXME in the code and I’m all ears to anyone that can help me
with that.  This option takes exactly two strings and should be allowed
anywhere on the command line.  Such that "llvm-nm -s __TEXT __text foo.o"
would work. But that does not as the CommandLine Library does not have a
way to make this work as far as I can tell.  For now the "-s __TEXT __text"
has to be last on the command line.