[llvm-nm] Add --portability as alias for --format=posix

GNU nm supports this alias, so supporting it in llvm-nm makes it easier
to transition between the two.

Fixes https://bugs.llvm.org/show_bug.cgi?id=40002

Reviewed by: mstorsjo, rupprecht

Differential Revision: https://reviews.llvm.org/D56312

llvm-svn: 350522