Refactor how we decide which sections to sort.

This is a bit more verbose, but it has a few advantages.

The logic on what to do with special sections like .init_array is not
duplicated. Before we would need keep isKnownNonreorderableSection in
sync.

I think with this the call graph based sorting can be implemented by
"just" returning a new order from buildSectionOrder.

llvm-svn: 324744