[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.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@215598 91177308-0d34-0410-b5e6-96231b3b80d8
[mach-o] Fix stub generation to work for dylibs and bundles

Split up the CRuntimeFile into one part for output types that need an entry
point and another part for output types that use stubs.

Add file 'test/mach-o/Inputs/libSystem.yaml' for use by test cases that
use -dylib and therefore may now need the helper symbol in libSystem.dylib.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@215602 91177308-0d34-0410-b5e6-96231b3b80d8