Handle 0 sized sections like any other section.

This is a case where there is inconsistency among ELF linkers:

* The spec says nothing special about empty sections.
* BFD ld removes them.
* Gold handles them like regular sections.

We were outputting them but sometimes ignoring them. This would create
odd looking outputs where a rw section could be in a ro segment for example.

The bfd way of doing things is also strange for the case where a symbol
points to the empty section.

Now we match gold and what seems to be the intention of the spec.

llvm-svn: 251988