[llvm-nm] Don't error out on multiple occurrances of the -g/--external-only flag

GNU binutils nm doesn't error out on this, and some projects' build
systems can end up doing that in some cases. Allowing that seems like
a better target than trying to avoid user projects passing multiple
-g parameters to $NM.

Differential Revision: https://reviews.llvm.org/D39539

llvm-svn: 317301