[mach-o] set ordinal in n_desc for undefined symbols

Mach-o uses "two-level namespace" where each undefined symbols is associated
with a specific dylib. This means at runtime the loader (dyld) does need to
search all loaded dylibs for that symbol but rather just the one specified.

Now that llvm-nm -m prints out that info, properly set that info, and test
it in the hello world test cases.

llvm-svn: 215598