[ELF] Fix compareSections assertion failure when OutputDescs in sectionCommands are non-contiguous

In a `--defsym y0=0 -T a.lds` link where a.lds contains only INSERT
commands, the `script->sectionCommands` layout may be:
```
orphan sections
SymbolAssignment due to --defsym
sections created by INSERT commands
```

The `OutputDesc` objects are not contiguous in sortInputSections, and
`compareSections` will be called with a SymbolAssignment argument,
leading to an assertion failure.