SWAT-711: Create Mac app bundle symlinks only for .dylib files that exist.

Previous viewer_manifest.py unconditionally created Mac symlinks for all expected
.dylib files. Recent change to revert to statically linking llcommon means we
no longer build libllcommon.dylib, therefore we no longer copy it, therefore
any symlink to that library will be broken by definition.
Change to create symlinks for .dylib files that were successfully copied.